target-mips: fix updating XContext on mmu exception

Correct updating XContext.Region field on mmu exceptions.
If Config3.CTXTC = 0 then the R field of XContext has to be updated
with the value of bits 63..62 of the virtual address upon a TLB
exception.
Also fixed the below line which overs 80 characters.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>