target/mips: Implement segmentation control

Implement the optional segmentation control feature in the virtual to
physical address translation code.

The fixed legacy segment and xkphys handling is replaced with a dynamic
layout based on the segmentation control registers (which should be set
up even when the feature is not exposed to the guest).

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