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>