dt-bindings: pinctrl: document Broadcom Northstar pin mux controller

Northstar has mux controller just like Northstar Plus and Northstar2.
It's a bit different though (different registers & pins) so it requires
its own binding.

It's needed to allow other block bindings specify required mux setup.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/brcm,bcm4708-pinmux.txt

+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.
+
+Required properties:
+- compatible: must be one of:
+	"brcm,bcm4708-pinmux"
+	"brcm,bcm4709-pinmux"
+	"brcm,bcm53012-pinmux"
+- reg: iomem address range of CRU (Central Resource Unit) pin registers
+- reg-names: "cru_gpio_control" - the only needed & supported reg right now
+
+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:
+	pinctrl@1800c1c0 {
+		compatible = "brcm,bcm4708-pinmux";
+		reg = <0x1800c1c0 0x24>;
+		reg-names = "cru_gpio_control";
+
+		spi {
+			function = "spi";
+			groups = "spi_grp";
+		};
+	};