dt-bindings: pinctrl: Document 7211 compatible for brcm, bcm2835-gpio.txt

Document the brcm,bcm7211-gpio compatible string in the
brcm,bcm2835-gpio.txt document.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200531001101.24945-2-f.fainelli@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>