usb: dwc2: add 'mode' which based on Kconfig select or dts setting

According to the "dr_mode", the otg controller can work as
device role and host role. Some boards always want to use host mode
and some other boards want to use gadget mode. We use the dts setting
to set dwc2's mode, rather than fixing it to whatever hardware says.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
enable host only mode for veyron board

Change-Id: Ied8aa722af6e6720eb341585497eca7ec6e1e0d5
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>