arm64: pgtable: use a single bit for PTE_WRITE regardless of DBM

Depending on CONFIG_ARM64_HW_AFDBM, we use either bit 57 or 51 of the
pte to represent PTE_WRITE. Given that bit 51 is reserved prior to
ARMv8.1, we can just use that bit regardless of the config option. That
also matches what happens if a kernel configured with ARM64_HW_AFDBM=y
is run on a CPU without the DBM functionality.

Cc: Julien Grall <julien.grall@citrix.com>
Tested-by: Julien Grall <julien.grall@citrix.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>