Add /dev/gpiomem device for rootless user GPIO access

Signed-off-by: Luke Wren <luke@raspberrypi.org>

bcm2835-gpiomem: Fix for ARCH_BCM2835 builds

Build on ARCH_BCM2835, and fail to probe if no IO resource.

See: https://github.com/raspberrypi/linux/issues/1154