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 <[email protected]>
Reviewed-by: Yongbok Kim <[email protected]>
Cc: Aurelien Jarno <[email protected]>
[[email protected]:
  cosmetic changes]
Signed-off-by: Yongbok Kim <[email protected]>