boot/efi: factor out generic mmu functions

This change moves the common functions related to EFI memory map
to arch/generic for arm, arm64, x86.

riscv64 and x86_64 shall be handled separately as they use some
more architecture-specific logic:
* riscv64 needs special treatment for the M-mode resident code
located at 0x80000000 which is reported as available in the
EFI memory map provided by u-boot.
* x86_64 has a slightly different logic for building the
memory regions list so some additional rework and testing will be
needed before it can be unified with the generic mmu functions.

Change-Id: I430c84dfc693c5b6f04c170dec8ffb1db2c2ace1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5694
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>