xen/arm: initialize the GIC irq properties of interrupts routed to guests

xen/arm: initialize the GIC irq properties of interrupts routed to guests

We are currently initializing GIC irq properties (ITARGETSR, IPRIORITYR,
and GICD_ICFGR) only in gic_route_irq, that is not called for guest
interrupts at all.
Move the initialization into a separate function
(gic_set_irq_properties) and call it from gic_route_irq_to_guest.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>