usb: dwc3: of-simple: Add support to get resets for the device

Add support to get a list of resets available for the device.
These resets must be kept de-asserted until the device is
in use.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
[p.zabel@pengutronix.de: switch to hidden reset control array]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>