There is really no reason to use and pass around a struct when its only
member is the (fixed) base address.

Remove the struct and just use the base address on its own inside the
GPIO driver. Then set the base address automatically.

This simplifies GPIO setup for users, which now don't need to deal with
zeroing a struct and setting the base address anymore.

Change-Id: I3060f7859e3f8ef9a24cc8fb38307b5da943f127
Signed-off-by: Andre Przywara <andre.przywara@arm.com>