ARM: imx: move irq_domain_add_legacy call into avic driver

Move irq_domain_add_legacy call from imx27-dt.c into avic init function
and have the avic driver adopt irqdomain support for both DT and non-DT
boot.

Now avic init function calls irq_alloc_descs to get irq_base and adds
a lenacy irqdomain with the irq_base, so that the mapping between avic
irq and Linux irq number can be handled by irqdomain.

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>