arm64: clean up memmap handling in efi runtime services

The arm EFI stub passes a copy of the memmap to the kernel via an FDT
property but arm64 used an address and size because the memmap in the
FDT would not be guaranteed 64-bit alignment. This patch makes use of
the passed memmap by copying one memory descriptor at a time to a local
variable with proper alignment as needed. This actually simplifies the
handling by eliminating the need to map the memmap into kernel space.
It also opens up the possibility of common memmap handling code to be
used by multiple architectures.

Signed-off-by: Mark Salter <msalter@redhat.com>
Revert "EFI: add EFI memmap address and size to device tree"

This reverts commit c7a5fce35e342570d63b3371afa7478a1ae8985f.