Broadcom Northstar pins mux controller

Some of Northstar SoCs's pins can be used for various purposes thanks to the mux
controller. This binding allows describing mux controller and listing available
functions. They can be referenced later by other bindings to let system
configure controller correctly.

A list of pins varies across chipsets so few bindings are available.

Node of the pinmux must be nested in the CRU (Central Resource Unit) "syscon"
noce.

Required properties:
- compatible: must be one of:
	"brcm,bcm4708-pinmux"
	"brcm,bcm4709-pinmux"
	"brcm,bcm53012-pinmux"
- offset: offset of pin registers in the CRU block

Functions and their groups available for all chipsets:
- "spi": "spi_grp"
- "i2c": "i2c_grp"
- "pwm": "pwm0_grp", "pwm1_grp", "pwm2_grp", "pwm3_grp"
- "uart1": "uart1_grp"

Additionally available on BCM4709 and BCM53012:
- "mdio": "mdio_grp"
- "uart2": "uart2_grp"
- "sdio": "sdio_pwr_grp", "sdio_1p8v_grp"

For documentation of subnodes see:
Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

Example:
	dmu@1800c000 {
		compatible = "simple-bus";
		ranges = <0 0x1800c000 0x1000>;
		#address-cells = <1>;
		#size-cells = <1>;

		cru@100 {
			compatible = "syscon", "simple-mfd";
			reg = <0x100 0x1a4>;

			pinctrl {
				compatible = "brcm,bcm4708-pinmux";
				offset = <0xc0>;

				spi-pins {
					function = "spi";
					groups = "spi_grp";
				};
			};
		};
	};