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So far the Raspberry Pi 3 build needs the GPIO driver just for BL2.
Upcoming changes will require some GPIO code in BL1 and BL31 also, so
move those driver files into the common source section.

This does not affect BL31 code size at all, and bl1.bin just increases
by 144 bytes, but doesn't affect the padded binary size at all.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I7639746dc241c1e69099d85d2671c65fa0108555