arm64: gic: disable the ITS if it's enabled prior to configuration

The ITS is defined to be disabled on a warm reset, but we may be coming
in via another kernel/hypervisor type setup where the ITS has been
previously configured then relinquished to the next kernel in the chain.

If it's enabled, the later configuration of GITS_BASER will almost
certainly fail -- clear it to prevent that.

Reviewed by: andrew
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34546