[MIPS] malta_int.c: make 4 variables static

The following variables defined in arch/mips/mips-boards/malta/malta_int.c
can become static: msc_irqmap[], msc_nr_irqs, msc_eicirqmap[], and
msc_nr_eicirqs. This patch makes them static.

Successfully build-tested using default configs for Malta, Atlas
and SEAD boards.

Runtime test successfully performed by booting the Malta 4Kc board
up to the shell prompt.

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