This gives TUSB host side enumeration support for low speed devices (like
USB mice), through a Mini-A connector that's not removed ... basically, a
non-OTG configuration, with the ID pin always grounded.

Basically it punts the "turn VBUS power on/off" to board-specific logic,
and implements it for TUSB using software switching (rather than having
the controller do it).

There are various issues, notably

    (a)	remote wakeup not getting passed to the root hub then down
	to the device (mouse) that issued the wakeup;

    (b)	strange "vbus error" reports on device connection if nothing
    	is hooked up at driver initialization; and

    (c) full or high speed devices see spurious disconnect events
	right after they've been reset (hw bug?);

Includes various small cleanups too, notably starting to obey the OTG
state machine and use the OTG timer.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>