MIPS: GIC: Move GIC_NUM_INTRS into platform irq.h

The value of GIC_NUM_INTRS is platform-specific. Using a default value
from gic.h will result in incorrect behaviour on some systems, so
require a suitable definition to be present in the platform's irq.h.

Signed-off-by: Jeffrey Deans <jeffrey.deans@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7373/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>