mips: more fixes to the MIPS interrupt glue logic

Commit 36388314 moved most of the
interrupt logic to cpu-exec.c. Remove the remaining useless code
and fix software interrupts.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Acked-by: Edgar E. Iglesias <edgar@axis.com>
Tested-by: Edgar E. Iglesias <edgar@axis.com>