Documentation: Add APM X-Gene SoC 15Gbps Multi-purpose PHY driver binding documentation

* APM X-Gene 15Gbps Multi-purpose PHY nodes

PHY nodes are defined to describe on-chip 15Gbps Multi-purpose PHY. Each

PHY (pair of lanes) has its own node.

Required properties:

- compatible		: Shall be "apm,xgene-phy".

- reg			: PHY memory resource is the SDS PHY access resource.

- #phy-cells		: Shall be 1 as it expects one argument for setting

			  the mode of the PHY. Possible values are 0 (SATA),

			  1 (SGMII), 2 (PCIe), 3 (USB), and 4 (XFI).

Optional properties:

- status		: Shall be "ok" if enabled or "disabled" if disabled.

			  Default is "ok".

- clocks		: Reference to the clock entry.

- apm,tx-eye-tuning	: Manual control to fine tune the capture of the serial

			  bit lines from the automatic calibrated position.

			  Two set of 3-tuple setting for each (up to 3)

			  supported link speed on the host. Range from 0 to

			  127 in unit of one bit period. Default is 10.

- apm,tx-eye-direction	: Eye tuning manual control direction. 0 means sample

			  data earlier than the nominal sampling point. 1 means

			  sample data later than the nominal sampling point.

			  Two set of 3-tuple setting for each (up to 3)

			  supported link speed on the host. Default is 0.

- apm,tx-boost-gain	: Frequency boost AC (LSB 3-bit) and DC (2-bit)

			  gain control. Two set of 3-tuple setting for each

			  (up to 3) supported link speed on the host. Range is

			  between 0 to 31 in unit of dB. Default is 3.

- apm,tx-amplitude	: Amplitude control. Two set of 3-tuple setting for

			  each (up to 3) supported link speed on the host.

			  Range is between 0 to 199500 in unit of uV.

			  Default is 199500 uV.

- apm,tx-pre-cursor1	: 1st pre-cursor emphasis taps control. Two set of

			  3-tuple setting for each (up to 3) supported link

			  speed on the host. Range is 0 to 273000 in unit of

			  uV. Default is 0.

- apm,tx-pre-cursor2	: 2st pre-cursor emphasis taps control. Two set of

			  3-tuple setting for each (up to 3) supported link

			  speed on the host. Range is 0 to 127400 in unit uV.

			  Default is 0x0.

- apm,tx-post-cursor	: Post-cursor emphasis taps control. Two set of

			  3-tuple setting for Gen1, Gen2, and Gen3. Range is

			  between 0 to 0x1f in unit of 18.2mV. Default is 0xf.

- apm,tx-speed		: Tx operating speed. One set of 3-tuple for each

			  supported link speed on the host.

			   0 = 1-2Gbps

			   1 = 2-4Gbps (1st tuple default)

			   2 = 4-8Gbps

			   3 = 8-15Gbps (2nd tuple default)

			   4 = 2.5-4Gbps

			   5 = 4-5Gbps

			   6 = 5-6Gbps

			   7 = 6-16Gbps (3rd tuple default)

NOTE: PHY override parameters are board specific setting.

Example:

		phy1: phy@1f21a000 {

			compatible = "apm,xgene-phy";

			reg = <0x0 0x1f21a000 0x0 0x100>;

			#phy-cells = <1>;

			status = "disabled";

		};

		phy2: phy@1f22a000 {

			compatible = "apm,xgene-phy";

			reg = <0x0 0x1f22a000 0x0 0x100>;

			#phy-cells = <1>;

			status = "ok";

		};

		phy3: phy@1f23a000 {

			compatible = "apm,xgene-phy";

			reg = <0x0 0x1f23a000 0x0 0x100>;

			#phy-cells = <1>;

			status = "ok";

		};