Enable use of ofwcons for early debug

This change enables the use of OpenFirmware Console (ofwcons), even when VGA is
available, allowing early kernel messages to be seen, that is important in case
of crashes before VGA console initialization.

This is specially useful in virtualized environments, where the user/developer
doesn't have full control of the virtualization engine (e.g. OpenStack).

The old behavior is preserved by default and, in order to use ofwcons, a few
tunables that have been introduced need to be set:

• hw.ofwfb.disable=1 - disable OFW FrameBuffer device
• machdep.ofw.mtx_spin=1 - change PPC OFW mutex to SPIN type, to match kernel console's mutex type
• debug.quiesce_ofw=0 - don't call OFW quiesce, needed to keep ofwcons I/O working

More details can be found at differential revision D20640.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D20640