ARM: SAMSUNG: move driver strength gpio configuration helper to common dir

Driver strength parameter can be changed not only on S5PC100 but also
on S5PV210/S5PC110 platforms, so move the helper functions to the common
plat-samsung directory.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>