usb: musb: Implement tracing for state change events

usb: musb: Implement tracing for state change events

The devctl register on musb is the only way to get state information
on musb. The hardware can easily get confused because it tries to do
things on it's own automagically, and things like slow VBUS rise can
make things fail.

Let's make it easier to debug the ongoing state change issues that
keep popping up on regular basis and add tracing support.

With these changes we can easily trace musb state change events with:

echo 1 > /sys/kernel/debug/tracing/events/musb/musb_state/enable
cat /sys/kernel/debug/tracing/trace_pipe
echo 0 > /sys/kernel/debug/tracing/events/musb/musb_state/enable

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Andreas Kemnade <andreas@kemnade.info>
Cc: Bhushan Shah <bshah@kde.org>
Cc: Drew Fustini <drew@beagleboard.org>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20210604080536.12185-2-tony@atomide.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>