irqchip/mmp: Add missing chained_irq_{enter,exit}()

The lack of chained_irq_exit() leaves the muxed interrupt masked on MMP3.
For reasons unknown this is not a problem on MMP2.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20190822092643.593488-8-lkundrak@v3.sk