Mark gpio as "optional" in files.mv and edit Marvell's kernconfs

Including arm/mv/gpio.c now depends on 'gpio' device. 'device gpio' was
added to all kernconf files of Marvell boards, except ARMADAXP (dummy
mv_gpio_res definition was removed) and ARMADA38X (not supported yet).

This commit allows to use generic files.mv on A38X.

Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4372