x86/platform/UV: Bring back the call to map_low_mmrs in uv_system_init

A while back the following commit:

  d394f2d9 ("x86/platform/UV: Remove EFI memmap quirk for UV2+")

changed uv_system_init() to only call map_low_mmrs() on older UV1 hardware,
which requires EFI_OLD_MEMMAP to be set in order to boot.

The recent changes to the EFI memory mapping code in:

  d2f7cbe7 ("x86/efi: Runtime services virtual mapping")

exposed some issues with the fact that we were relying on the EFI memory
mapping mechanisms to map in our MMRs for us, after commit d394f2d9

.

Rather than revert the entire commit and go back to forcing
EFI_OLD_MEMMAP on all UVs, we're going to add the call to map_low_mmrs()
back into uv_system_init(), and then fix up our EFI runtime calls to use
the appropriate page table.

For now, UV2+ will still need efi=old_map to boot, but there will be
other changes soon that should eliminate the need for this.

Signed-off-by: Alex Thorlton <athorlton@sgi.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Russ Anderson <rja@sgi.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Link: http://lkml.kernel.org/r/1462401592-120735-1-git-send-email-athorlton@sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>