src.opts.mk: sync FDT and OFED options with kern.opts.mk

Where options exist in both src.opts.mk and kern.opts.mk they should be
kept in sync.

It may be that the option should be flipped the other way (e.g.,
enabling OFED in the kernel on arm64); if so that will be done as a
subsequent commit after further testing.

PR: 269994
Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38952