rockchip: board: Remove dwc3 usb init and gadget handler functions

Remove board_usb_init() and dm_usb_gadget_handle_interrupts() functions
related to dwc3, they use e.g. a hard-coded reg address for RK3399 and
are obsolete with use of DM_USB_GADGET.

Use of DM_USB_GADGET, USB_DWC3_GENERIC and USB_DWC3_GADGET have replaced
same feature provided by the removed functions on RK3399 boards.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>