ARM: ixp4xx: Convert to SPARSE_IRQ

This localizes the <mach/irqs.h> header to the mach-ixp4xx
directory, removes NR_IRQS and switches IXP4xx over to using
SPARSE_IRQ.

This is a prerequisite for DT support.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>