gpio/samsung: use ioremap() for EXYNOS4 GPIOlib

This patch changes to use ioremap() for EXYNOS4210
so that we can drop the static mapping for EXYNOS
SoCs.

Note: Will be updated for all of Samsung GPIOlibs
to use ioremap() next time.

Signed-off-by: Sangsu Park <sangsu4u.park@samsung.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>