irqchip/gic-v3-its: Generalize LPI configuration

We're are going to need to change a bit more than just the enable
bit in the LPI property table in the future. So let's change the
LPI configuration funtion to take a set of bits to be cleared,
and a set of bits to be set.

This way, we'll be able to use it when a guest updates an LPI
property (priority, for example).

Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>