gicv3_its: do LPI init only once per CPU

The initialization required for LPIs (setting up pending tables etc.)
has to be done just once per CPU, even in the case where there are
multiple ITS blocks associated with the CPU.

Add a flag lpi_enabled in the per-cpu distributor info for this and
use it to ensure that we call its_init_cpu_lpi() just once.

This enables us to support platforms where multiple GIC ITS blocks
can generate LPIs to a CPU.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D19844