usb: dwc3: document usb_psy in struct dwc3

The new struct member was added to struct dwc3, but
a documentation was missing:

drivers/usb/dwc3/core.h:1273: warning: Function parameter or member 'usb_psy' not described in 'dwc3'

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ray Chi <raychi@google.com>
Link: https://lore.kernel.org/r/20210303095802.2801733-1-raychi@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>