Currently, enable_mmu_el1() or enable_mmu_el3() is kept outside the
common function because the appropriate one must be chosen.

Use enable_mmu() and move it to the common function.

Change-Id: If2fb651691a7b6be05674f5cf730ae067ba95d4b
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>