efi: memmap: Move manipulation routines into x86 arch tree

The EFI memory map is a description of the memory layout as provided by
the firmware, and only x86 manipulates it in various different ways for
its own memory bookkeeping. So let's move the memmap routines that are
only used by x86 into the x86 arch tree.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>