rockchip: Add USB to the default boot targets

Now that most rockchip SoC based board have usb host support, enable
USB boot targets by default.

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Fixed build errors when CONFIG_CMD_USB not defined:
Signed-off-by: Simon Glass <sjg@chromium.org>