bcma: use absolute base for SoC GPIO pins

On some BCM5301x ARM devices, user space still needs to control some
system GPIO pins for which no driver exists. This is a lot easier to do
with a predictable GPIO base.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>