MIPS: Malta: Use new GIC accessor functions

Use the accessor functions provided by asm/mips-gic.h rather than
calling functions provided by the GIC irqchip driver, in preparation for
those non-IRQ-related functions being removed from the irqchip driver.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>