ARM: imx: add a legacy irqdomain for mx31ads

Call irq_alloc_descs to get the irq_base for mx31ads, and add a legacy
irqdomain using the irq_base, so that the mapping between mx31ads
hardware irq and Linux irq number can be dynamically handled by
irqdomain.  As the result, the use of MXC_BOARD_IRQ_START can be
completely removed from mach-mx31ads.c.

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