irqchip: Add COMPILE_TEST support for IMX_INTMUX

Add COMPILE_TEST support to IMX_INTMUX driver for better compile
testing coverage.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/1583588547-7164-1-git-send-email-Anson.Huang@nxp.com