ARM: exynos: prepare for sparse IRQ

When we enable CONFIG_SPARSE_IRQ, we have to set the value of NR_IRQS in
the machine_desc for legacy IRQ domains, and any file referring to the
number of interrupts or a specific number must include the mach/irqs.h
header file explicitly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>