usb: rockchip: add rockusb command

this patch add rockusb command. the usage is
rockusb <USB_controller> <devtype> <dev[:part]>
e.g. rockusb 0 mmc 0

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Makefile b/cmd/Makefile
index ab45bf4..ce65cef 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -105,6 +105,7 @@
 obj-$(CONFIG_CMD_REGINFO) += reginfo.o
 obj-$(CONFIG_CMD_REISER) += reiser.o
 obj-$(CONFIG_CMD_REMOTEPROC) += remoteproc.o
+obj-$(CONFIG_CMD_ROCKUSB) += rockusb.o
 obj-$(CONFIG_SANDBOX) += host.o
 obj-$(CONFIG_CMD_SATA) += sata.o
 obj-$(CONFIG_CMD_NVME) += nvme.o