ARM: exynos: remove custom .init_time hook

With arch/arm calling of_clk_init(NULL) from time_init(), we can now
remove custom .init_time hooks. While at it, also remove some now
redundant includes.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>