Move if_smc_fdt.c to live in sys/dev/smc. It's not specific to the ARM
Versatile hardware.
o Add sysctls to enable/disable potentially dengerous key combinations, like
        reboot/halt/debug.
o Add support for most key combinations supported by syscons(4).

Reviewed by:	dumbbell, emaste (prev revision of D747)
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation