irqchip/gic-v3-its: Honor hypervisor enforced LPI range

A recent extension to the GIC architecture allows a hypervisor to
arbitrarily reduce the number of LPIs available to a guest, no
matter what the GIC says about the valid range of IntIDs.

Let's factor in this information when computing the number of
available LPIs

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>