usb: dwc2: Move gadget settings into core_params

Move the gadget devicetree settings into the core_params structure and
document them. Then set and check them in params.c, with the addition of
some helper functions, and remove the equivalent code in gadget.c.

Because these parameters came from the standalone s3c driver, they have
a fixed default value rather than an autodetected one. Preserve and
document this behavior to avoid any compatibility issues.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>