* 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>;
};