powerpc/qe: Select QE_USB with USB_GADGET_FSL_QE

Boards should know when QE_USB is used, so that they can configure USB
clocks and pins.

Another option would be to add 'select QE_USB' into USB_GADGET_FSL_QE,
but selects are evil since they don't support dependencies.

While at it, also remove 'host' from the symbol description, since the
QE_USB code is used to support the gadget driver as well.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>