target-openrisc: Correct memory bounds checking for the tlb buffers

The mtspr and mfspr routines didn't check for the correct memory boundaries.
This fixes a segmentation fault while booting Linux.

Signed-off-by: Sebastian Macke <sebastian@macke.de>
Reviewed-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Jia Liu <proljc@gmail.com>
