Add support for MSI/MSIX deallocation on GICv3-ITS

Allow to deallocate previously allocated ITS device along with
its interrupts. Interrupt numbers are being freed when the last
LPI number is no longer busy.

Reviewed by: wma
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D6351