irqchip/gic-v3-its: Move LPI definitions around

The various LPI definitions are in the middle of the code, and
would be better placed at the beginning, given that we're going
to use some of them much earlier.

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