MIPS: lantiq: add ipi handlers to make vsmp work

Add IPI handlers to the interrupt code. This patch makes MIPS_MT_SMP work
on lantiq socs. The code is based on the malta implementation.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3704/


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