drivers: gpio: Add GPIO driver for BCM2711

The BCM2711 SoC exposes 58 GPIOs. The first 28 (bank 0) are accessible
to users via the 40-pin header, while the others (bank 1) are used for
controlling on-board peripherals.

This also update doc of `rpi_4b` board.

Signed-off-by: Chen Xingyu <hi@xingrz.me>