usb: ulpi: Clean up how we enable support

usb: ulpi: Clean up how we enable support

The way we enable ULPI support today isn't something that should work.
The "optional" keyword in a choice statement is not a documented
feature. To make this work in a supported way, make USB_ULPI something
we ask about if USB_HOST is set. Next, we move the choice of what
viewer to use to be after the framework portion and to depend on that.
We then borrow a few words from the top-level README to make the help
text here clearer. Finally we make the Qualcomm driver select ULPI as
it's required and we make the tegra driver not duplicate a check that
Kconfig now handles for us.

Signed-off-by: Tom Rini <trini@konsulko.com>