MIPS: Fix build error when SMP is used without GIC

The MIPS_GIC_IPI should only be selected when MIPS_GIC is also
selected, otherwise it results in a compile error. smp-gic.c uses some
functions from include/linux/irqchip/mips-gic.h like
plat_ipi_call_int_xlate() which are only added to the header file when
MIPS_GIC is set. The Lantiq SoC does not use the GIC, but supports SMP.
The calls top the functions from smp-gic.c are already protected by
some #ifdefs

The first part of this was introduced in commit 72e20142

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

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: stable@vger.kernel.org # v3.15+
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12774/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>