devicetree: phy: rockchip-emmc: pulldown property

Update the documentation and add the bool property
enable-strobe-pulldown used to enable the internal pull-down for the
strobe line.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Link: https://lore.kernel.org/r/20201129054416.3980-3-chris.ruehl@gtsys.com.hk


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