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 8b5c2b45b8f0a ("phy: rockchip: set pulldown for strobe line in dts")
commit a8cef928276bb ("phy: rockchip-emmc: output tap delay dt property")

Signed-off-by: Chris Ruehl <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/phy/rockchip-emmc-phy.txt

···
  - drive-impedance-ohm: Specifies the drive impedance in Ohm.
                         Possible values are 33, 40, 50, 66 and 100.
                         If not set, the default value of 50 will be applied.
- - enable-strobe-pulldown: Enable internal pull-down for the strobe line.
-                           If not set, pull-down is not used.
- - output-tapdelay-select: Specifies the phyctrl_otapdlysec register.
-                           If not set, the register defaults to 0x4.
-                           Maximum value 0xf.
+ - rockchip,enable-strobe-pulldown: Enable internal pull-down for the strobe
+                                    line.  If not set, pull-down is not used.
+ - rockchip,output-tapdelay-select: Specifies the phyctrl_otapdlysec register.
+                                    If not set, the register defaults to 0x4.
+                                    Maximum value 0xf.
 
 Example: