arm64: mm: make use of new memblocks_present() helper

Cleanup the arm64_memory_present() function seeing it's very
similar to other arches.

memblocks_present() is a direct replacement of arm64_memory_present()

Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>