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

A while back the following commit:

  d394f2d9d8e1 ("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:

  d2f7cbe7b26a ("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 d394f2d9d8e1.

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>