MIPS: Decode c0_config4 for large TLBs.

For processors that have more than 64 TLBs, we need to decode both
config1 and config4 to determine the total number TLBs.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/866/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>