devicetree: phy: rockchip-emmc optional add vendor prefix

Update the documentation and add the vendor prefix to the optional
properties referred in vendor-prefixes.yaml.
Follow up with
commit 8b5c2b45 ("phy: rockchip: set pulldown for strobe line in dts")
commit a8cef928 ("phy: rockchip-emmc: output tap delay dt property")

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201215014409.905-2-chris.ruehl@gtsys.com.hk


Signed-off-by: Vinod Koul <vkoul@kernel.org>