rockchip: board: Prepare for use of DM_USB_GADGET with DWC2_OTG

The board_usb_init() and board_usb_cleanup() functions is always
included when USB_GADGET and USB_GADGET_DWC2_OTG is enabled.

Prepare for a change to use DM_USB_GADGET with DWC2_OTG by adding an
extra ifdef condition. The extra separate ifdef for USB_GADGET prepare
for next patch that adds a g_dnl_bind_fixup() function.

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