dt-bindings: pinctrl: bcm2835-gpio: Add generic pinctrl support

Added generic pin configuration and multiplexing support,
and should be preferred than brcm legacy one.

Signed-off-by: Matheus Castello <matheus@castello.eng.br>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>