dt-bindings: add bindings doc for ZTE pinctrl

* ZTE ZX Pin Controller

The pin controller on ZTE ZX platforms is kinda of hybrid.  It consists of

a main controller and an auxiliary one.  For example, on ZX296718 SoC, the

main controller is TOP_PMM and the auxiliary one is AON_IOCFG.  Both

controllers work together to control pin multiplexing and configuration in

the way illustrated as below.

           GMII_RXD3 ---+

                        |

             DVI1_HS ---+----------------------------- GMII_RXD3 (TOP pin)

                        |

             BGPIO16 ---+                               ^

                                                        | pinconf

                        ^                               |

                        | pinmux                        |

                        |                               |

                   TOP_PMM (main)           AON_IOCFG (aux)

                        |                       |       |

                        |                pinmux |       |

                        | pinmux                v       |

                        v                               | pinconf

                                    KEY_ROW2 ---+       v

        PORT1_LCD_TE ---+                       |

                        |            AGPIO10 ---+------ KEY_ROW2 (AON pin)

          I2S0_DOUT3 ---+                       |

                        |-----------------------+

            PWM_OUT3 ---+

                        |

             VGA_VS1 ---+

For most of pins like GMII_RXD3 in the figure, the pinmux function is

controlled by TOP_PMM block only, and this type of pins are meant by term

'TOP pins'.  For pins like KEY_ROW2, the pinmux is controlled by both

TOP_PMM and AON_IOCFG blocks, as the available multiplexing functions for

the pin spread in both controllers.  This type of pins are called 'AON pins'.

Though pinmux implementation is quite different, pinconf is same for both

types of pins.  Both are controlled by auxiliary controller, i.e. AON_IOCFG

on ZX296718.

Required properties:

- compatible: should be "zte,zx296718-pmm".

- reg: the register physical address and length.

- zte,auxiliary-controller: phandle to the auxiliary pin controller which

  implements pinmux for AON pins and pinconf for all pins.

The following pin configuration are supported. Please refer to

pinctrl-bindings.txt in this directory for more details of the common

pinctrl bindings used by client devices.

- bias-pull-up

- bias-pull-down

- drive-strength

- input-enable

- slew-rate

Examples:

iocfg: pin-controller@119000 {

	compatible = "zte,zx296718-iocfg";

	reg = <0x119000 0x1000>;

};

pmm: pin-controller@1462000 {

	compatible = "zte,zx296718-pmm";

	reg = <0x1462000 0x1000>;

	zte,auxiliary-controller = <&iocfg>;

};

&pmm {

	vga_pins: vga {

		pins = "KEY_COL1", "KEY_COL2", "KEY_ROW1", "KEY_ROW2";

		function = "VGA";

	};

};

&vga {

	pinctrl-names = "default";

	pinctrl-0 = <&vga_pins>;

	status = "okay";

};