gadget: Select USB_GADGET_MANUFACTURER for rockchip

Select the gadget manufacturer as 'Rockchip' for
all rockchip platform SoC's.

This eventually drop the explicit defined added
on 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>