dt-bindings: pinctrl: bcm2835-gpio: Document BCM7211 compatible

BCM7211 has a slightly different block layout and some additional GPIO
registers that were added, document the compatible string.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/brcm,bcm2835-gpio.txt

@@ -5,6 +5,9 @@ controller, and pinmux/control device.
 
 Required properties:
 - compatible: "brcm,bcm2835-gpio"
+- compatible: should be one of:
+  "brcm,bcm2835-gpio" - BCM2835 compatible pinctrl
+  "brcm,bcm7211-gpio" - BCM7211 compatible pinctrl
 - reg: Should contain the physical address of the GPIO module's registers.
 - gpio-controller: Marks the device node as a GPIO controller.
 - #gpio-cells : Should be two. The first cell is the pin number and the