i.MX: Add FIQ support

Drivers which are going to use it will have to select it and use
imx_set_irq_fiq() to set FIQ mode for this interrupt.

Changes since V1:
- removed '#ifdef CONFIG_FIQ' from irqs.h
- renamed imx_set_irq_type() to imx_set_irq_fiq()
- made irqt unsigned
- fixed irq number check

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>