arm64: mm: Define arch_get_mmap_end, arch_get_mmap_base

Now that we have DEFAULT_MAP_WINDOW defined, we can arch_get_mmap_end
and arch_get_mmap_base helpers to allow for high addresses in mmap.

Signed-off-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>