ofwdump: install dependent on MK_FDT

The utility depends on the presence of the openfirm(4) pseudo-device,
which is gated by the FDT kernel option. The MK_FDT knob is correctly
set to "yes" for FDT/OFW-enabled platforms (powerpc* included); use it
to install the utility and eliminate the arch-specific Makefile logic.

No functional change intended.

Reviewed by: emaste, pkubaj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41977