arm64/gicv3: Skip programming vPE GITS_BASERs to avoid a boot crash

On AmpereOne, the current programming of GITS_BASER2 to individual
allocations per ITS causes a:

panic: APEI Fatal Hardware Error!

This is due to the ITS reporting a BASER2_DATA_ERR error. The GIC-700
expects all GITS_BASER2.Physical_Address fields to match system-wide.

Skip programming any vPE GITS_BASER registers to avoid this crash.

Sponsored by: Ampere Computing LLC
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D46094