MIPS: cpu: Add new cpu option for Hardware Table Walker.

Moreover, report hardware page table walker support as 'htw' in the ASE
list of /proc/cpuinfo, if the core implements this feature.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7334/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>