SB1 cache exception handling.

    
Expand SB1 cache error handling by adding SB1_CEX_ALWAYS_FATAL and
SB1_CEX_STALL, allowing configurable behavior on cache errors.
    
Signed-Off-By: Andy Isaacson <adi@broadcom.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>