rockchip:usb: add a simple readme for rockusb

add a simple readme to introduce rockusb and tell people how to use it

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Reviewed-by: Simon Glass <s

rockchip:usb: add a simple readme for rockusb

add a simple readme to introduce rockusb and tell people how to use it

Signed-off-by: Eddie Cai <eddie.cai.linux@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Changes in v7:
-none

Changes in v6:
-none

Changes in v5:
-none

Changes in v4:
-add some blank line to make it look better

Changes in v3:
-fix checkpatch error

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