On ThunderX2 we need to be careful to only map the memory the firmware
lists in the EFI memory map. As such we need to reduce the mappings to
restrict them to not be the full 1G block. For now reduce this to a 2M
block, however this may be further restricted to be 4k page aligned as
other SoCs may require.

This allows ThunderX2 to boot reliably to userspace without performing
any speculative memory accesses to invalid physical memory.

This is a recommit of r334035 now that we can access the EFI Runtime data
through the DMAP region.

Tested by: tuexen
Sponsored by: DARPA, AFRL