ARM: EXYNOS: Use exynos_init_io() as map_io callback

Since there is no board specific mapping needed on Exynos,
exynos_init_io() can be simplified and used as map_io callback for both
Exynos4 and Exynos5.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>