Create proper FDT attachment for GICv2m

Avoid probing GICv2m to any parent bus/driver. Instead, match
GICv2m driver with FDT complatible strings as not every GIC
has a MSI controller in the form of GICv2m extension.

Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Obtained from: Semihalf
Sponsored by: Cavium
Differential Revision: https://reviews.freebsd.org/D5015