MIPS: Move GIC IPI functions out of smp-cmp.c

The GIC IPI functions aren't necessarily specific to the "CMP
framework" SMP implementation, and will be used elsewhere in a
subsequent commit. This patch adds cleaned up GIC IPI functions to a
separate file which is compiled when a new CONFIG_MIPS_GIC_IPI Kconfig
symbol is selected, and selects that symbol for CONFIG_MIPS_CMP.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6359/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>