Use GIC-specific decoding function in mv_common.c

Add gic_decode_fdt function to fdt_pic_table, allowing to recognize GIC
interrupts on Armada38x. SOC_MV_ARMADA38X ifdef is required because A38X
is the only Marvell's platform in FreeBSD using GIC; lack of ifdef would
lead to linking errors on other platforms.

Reviewed by: andrew, ian, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Michal Stanek <mst@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4214