arm64/gicv3: Check if the hardware supports LPIs

Some simulators have the ITS in the DTB passed to the kernel, however
it is a runtime configuration option to enable it.

Check the GICD_TYPER register to see if LPIs are enabled before
attaching the ITS driver.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44914