gadget: Select USB_GADGET_VENDOR_NUM for rockchip

Gadget vendor number, 0x2207 is common across all platfroms
supported in Rockchip SoC.

So, select the same number globally, if ARCH_ROCKCHIP.

This eventually drop the explicit configs defined in supported
board defconfig files.

Tested-by: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>