usb: musb: remove ti81xx pieces from musb

ti81xx does not have a baseport mainline i.e. it should not boot. The
amount of rework that is required makes it easier to simply remove that
platform (i.e. that possible platform device) and add it later once it
comes back with DT support.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>