efi: Always map the UEFI memory map, even if runtime services are disabled

On arm64 ACPI's Error Interface makes use of the UEFI runtime map to learn
the memory attributes for the error status buffers.
(See arch_apei_get_mem_attribute())

If the memory map is not available because runtime services was disabled
at boot, then it isn't possible to learn the correct memory attributes
for these regions and 'device memory' gets chosen as the accidental default.

Juggle the code in arm_enable_runtime_services() around so that the memory
map is mapped even if runtime services are disabled.

Signed-off-by: James Morse <james.morse@arm.com>