bcm2835_gpio: Implement GPIOF_FUNC

So far we could only tell the gpio framework that a GPIO was mapped as input or
output, not as alternative function.

This patch adds support for determining whether a function is mapped as
alternative.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>