arm64: Move post_ttbr_update_workaround to C code

We will soon need to invoke a CPU-specific function pointer after changing
page tables, so move post_ttbr_update_workaround out into C code to make
this possible.

Change-Id: I8031daeddbce05538d12c7a67ef17a880ffc3dcb
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Patch-mainline: linux-arm-kernel @ 01/08/18, 9:32
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>