ETM: start cleaning up ETM_CTRL bit handling

ETM: start cleaning up ETM_CTRL bit handling

Provide better comments for the ETM_CTRL bits; use the correct bit
for half/full clock mode; and define a few more of the bits available
from the earliest ETM versions.

The new bit defintions use ETM_CTRL_* names to match their register
(instead of ETM_PORT_* or ETMV1_*).  For clarity, and better matching
to docs, they are defined with bitshifting not pre-computed masks.

Stop abusing typdefs for ETM_CTRL values; such values are not limited
to the enumerated set of individual bit values.

Rename etm->portmode to etm->control ... and start morphing it into a
single generic shadow of ETM_CTRL.  Eventually etm->tracemode should
vanish, so we can just write etm->control to ETM_CTRL.

Restore an "if" that somehow got dropped.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>