Allow to bootstrap by doing pkg add ./a/path/to/a/pkg_package.txz

Requested by:	many
MFC after:	3 days
Revert addition of sbintime and getsbintime that crept into r256963.

Pointyhat:	brooks
If the device tree directly contains the timebase frequency, use it. This
property is required by ePAPR, but maintain the fallback to bus-frequency
for compatibility.

MFC after:	2 weeks
A quick addendum: the standard says that timebase-frequency can be either
32 or 64 bits, so allow either.
Minor (mostly cosmetical) addition to r256960.