ARM: EXYNOS: Remove PM initcalls and useless indirection

This patch simplifies Exynos PM initialization and makes it
multiplatform friendly by replacing initcalls used originally to invoke
all the initialization code with explicit function calls.

In addition, an useless subsys_interface is removed, as all its .add_dev
callback did was setting two function pointers.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>