irqchip/mvebu-gicp: Use level-triggered MSIs between ICU and GICP

The ICU and GICP drivers are using an ugly side-band mechanism to
find out about the "clear" doorbell when using level interrupts.

Let's convert it to level-triggered MSIs, which result in a nice
cleanup.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Link: https://lkml.kernel.org/r/20180508121438.11301-4-marc.zyngier@arm.com