arm64: move update_mmu_cache() into asm/pgtable.h

Mark Brown reported an allnoconfig build failure in -next:

  Today's linux-next fails to build an arm64 allnoconfig due to "mm:
  make GUP handle pfn mapping unless FOLL_GET is requested" which
  causes:

  >       arm64-allnoconfig
  > ../mm/gup.c:51:4: error: implicit declaration of function
    'update_mmu_cache' [-Werror=implicit-function-declaration]

Fix the error by moving the function to asm/pgtable.h, as is the case
for most other architectures.

Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>