ARM i.MX avic: convert to use generic irq chip

Convert i.MX avic irq handler to use generic irq chip. This not only
provides a cleanup implementation of irq chip handler, but also
implements suspend/resume interface with the help of generic irq chip
interface.

Change mxc_irq_chip to a new structure mxc_extra_irq to handle fiq
and priority functions.

Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>