Platform/RPi: Don't describe MMIO regions as memory

When using ACPI OpRegions to poke device registers, Linux will use
the UEFI memory map to decide which memory attributes to use, and
so they should not be described as cacheable memory.

Since MMIO regions that don't require an OS virtual mapping at runtime
don't really belong in the UEFI memory map to begin with, omit them
entirely.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>