usb-musb: Add reset function

usb-musb: Add reset function

Add a separate reset function musb_reset() to the usb-musb interface,
so that users who implement a reset function can also reset usb-musb.
Use this in tusb6010.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
[Riku Voipio: Fixes and restructuring patchset]
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
[Peter Maydell: More fixes and cleanups for upstream submission]
Signed-off-by:  Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>