ARM: imx: add a legacy irqdomain for 3ds_debugboard

Call irq_alloc_descs to get the irq_base for 3ds_debugboard, and add
a legacy irqdomain using the irq_base, so that the mapping between
3ds_debugboard 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 3ds_debugboard.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>