usb: cdnsp: Device side header file for CDNSP driver

Patch defines macros, registers and structures used by
Device side driver.

Because the size of main patch is very big, I’ve decided to create
separate patch for cdnsp-gadget.h. It should simplify reviewing the code.
Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>