Platform: Add GPIO platform service for Musca-S1

On Musca-S1 GPIO is accessible only from the secure world
so a secure service is added for this platform to make it
accessible from the non-secure code as well.

Change-Id: I122c8fda72461bb9c7b20abc640183f552249766
Signed-off-by: Gabor Abonyi <gabor.abonyi@arm.com>