[MIPS] A few cleanups in malta_int.c

Both the fill_ipi_map() routine and the gic_intr_map array defined
in arch/mips/mips-boards/malta/malta_int.c are not used outside of
the latter file. Thus, these objects can become static. Moreover,
these two objects are used by the MT code only, which is why this
patch adds the appropriate ifdef.

While at it, this patch removes an unnecessary preprocessing macro
in favor of the commonly used ARRAY_SIZE.

Successfully tested using a Qemu-emulated Malta board for both SMP
and UP kernels.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>