arm64: Move cpu_resume_mmu to a head.S section

When turning on the mmu, there needs to be an identity
mapping of the current location as an intermediate
step. When CONFIG_STRICT_RWX is enabled, the .text
section gets pushed outside the original identity mapping.
Create an identity mapping for cpu_resume_mmu to ensure
this is always mapped.

Change-Id: I23f526e2828da548e273f266f15a303a4176a76d
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>