MIPS: GIC: Send IPIs using the GIC

If GIC is present, then use it to send IPIs between the cores.
Using GIC for IPIs is simpler and is usable for multicore
systems compared to the existing way of doing IPIs where all VPEs
had to be disabled for another VPE to access the Cause register
in one of the TCs and enable all the VPEs back.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6040/