arm64: Move post_ttbr_update_workaround to C code

arm64: Move post_ttbr_update_workaround to C code

commit 95e3de3590e3 upstream.

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.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>
Conflicts:
	don't include PAN related changes
	arch/arm64/include/asm/assembler.h
	arch/arm64/kernel/entry.S
	arch/arm64/mm/proc.S