usb: dwc3: Perform PHY reset before core reset

Some targets need to reset and initialize PHYs before performing
core reset. Add a device tree parameter to indicate this and
update the sequence accordingly.
Also add reset function to usb_phy.

Change-Id: I0865be8f4cc6c32d3c7dd240ade06126a4672469
Signed-off-by: Maya Erez <merez@codeaurora.org>