mxs: icoll: Add function to register an interrupt as FIQ source

MXS, unlike other ARM platforms, has no way to make a FIQ from an
interrupt from a driver, without poking directly into the icoll.

Add an exported function to do this.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Remove the need to poke into the interrupt controller

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>