MIPS: /proc/cpuinfo: always print the supported ISA

Currently the supported ISA is only printed on the latest architectures.
Print it also on legacy platforms.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6295/