ARM: imx: enable SPARSE_IRQ for imx platform

As all irqchips on imx have been changed to allocate their irq_descs,
and all unneeded mach/irqs.h inclusions on imx have been cleaned up,
now it's time to select SPARSE_IRQ for imx/mxc.

The SPARSE_IRQ support forces irqs allocation starting from 16.  All
those static irq number definition for SoCs need to shift 16 to keep
non-DT boot works.

With all those static IRQ number and start definitions removed from
mach/irqs.h, the header becomes just a container of a couple of
mach-imx specific irq/fiq calls.  Since mach/irqs.h is not included
by asm/irq.h now, the users of mxc_set_irq_fiq needs to explicitly
include mach/irqs.h themselves.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>