irqchip/gic: Unify GIC priority definitions

LPIs use the same priority value as other GIC interrupts.

Make the GIC default priority definition visible to ITS implementation
and use this same definition for LPI priorities.

Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>