xtensa: enable ARCH_HAS_DEBUG_VM_PGTABLE

xtensa kernels successfully build and run with
CONFIG_DEBUG_VM_PGTABLE=y, enable arch support for it.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>