target/mips: Add an MMU mode for ERL

target/mips: Add an MMU mode for ERL

The segmentation control feature allows a legacy memory segment to
become unmapped uncached at error level (according to CP0_Status.ERL),
and in fact the user segment is already treated in this way by QEMU.

Add a new MMU mode for this state so that QEMU's mappings don't persist
between ERL=0 and ERL=1.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
[yongbok.kim@imgtec.com:
  cosmetic changes]
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>