MIPS: Probe for and report hardware virtualization support.

The presence of the MIPS Virtualization Application-Specific Extension
is indicated by CP0_Config3[23].  Probe for this and report it in
/proc/cpuinfo.

Signed-off-by: David Daney <david.daney@cavium.com>
Patchwork: http://patchwork.linux-mips.org/patch/4904/
Signed-off-by: John Crispin <blogic@openwrt.org>