2nd try, after r289319:

HWPMC depends on pmu.c even if device pmu is not specified.
Would be great if we could just automatically enabled "device pmu"
if we try to compile in HWPMC.

Also several old kernel cnfigurations seem to have HWPMC enabled but are
pre-FDT and thus fail. So make pmu.c depend on fdt in case of hwpmc as
well.

MFC after: 2 weeks
Sponsored by: DARPA/AFRL
Differential Revision: https://reviews.freebsd.org/D3877