Documentation: Add doc for 8337 ethernet switch in sdxpoorwills

* Qualcomm Technologies Inc. SSDK Driver.

Add missing DT bindings documentation for 8337 ethernet switch.

Required properties:

- compatible: Should be "qcom,ess-switch-qca83xx"

- qcom,switch-access-mode: Should be "mdio" or "local bus"

- qcom,link-intr-gpio: Link interrupt number used by switch

- qcom,switch-cpu-bmp: Switch cpu port bitmap

- qcom,switch-lan-bmp: Switch lan port bitmap

- qcom,switch-wan-bmp: Switch wan port bitmap

- qcom,ar8327-initvals: Initial qca83xx configuration

Optional:

- qcom,link-polling-required: Boolean- Present if using polling for link check

Example:

ess-switch@0 {

	compatible = "qcom,ess-switch-qca83xx";

	qcom,switch-access-mode = "mdio";

	qcom,ar8327-initvals = <

		0x0000c 0x7600000   /* PAD6_MODE */

		0x00008 0x0         /* PAD5_MODE */

		0x000e4 0xaa545     /* MAC_POWER_SEL */

		0x000e0 0xc74164de  /* SGMII_CTRL */

		0x0007c 0x4e        /* PORT0_STATUS */

		0x00094 0x4e        /* PORT6_STATUS */

	>;

	qcom,link-intr-gpio = <2>;

	qcom,switch-cpu-bmp = <0x40>;    /* cpu port bitmap */

	qcom,switch-lan-bmp = <0x3e>;    /* lan port bitmap */

	qcom,switch-wan-bmp = <0x0>;     /* wan port bitmap */

};