ARM i.MX irq: Compile avic irq code only on SoCs that need it

This patch adds a Kconfig option for the avic irq controller
and lets the SoCs that need it select this option.
Also, as we have two irq controllers for i.MX, irq.c is not
appropriate anymore, so rename it to avic.c

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>