efi: Add efi_memmap_install() for installing new EFI memory maps

While efi_memmap_init_{early,late}() exist for architecture code to
install memory maps from firmware data and for the virtual memory
regions respectively, drivers don't care which stage of the boot we're
at and just want to swap the existing memmap for a modified one.

efi_memmap_install() abstracts the details of how the new memory map
should be mapped and the existing one unmapped.

Tested-by: Dave Young <dyoung@redhat.com> [kexec/kdump]
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> [arm]
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Peter Jones <pjones@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>