Simplify GICv3 related drivers' naming

Rename gic_v3_ instances to simply use 'gic' and 'its'.
The information about the controller's revision is printed
in the device announcement during boot anyway.
The intention behind this change is to avoid somewhat misleading
GIC instances naming such as:

gic_v30
gic_v31
...

etc.

Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5016