MIPS: lantiq: adds support for gptu timers

Lantiq socs have a General Purpose Timer Unit (GPTU). This driver allows us to
initialize the timers. The voice firmware needs these timers as a reference.

Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4236/