phy: rockchip: set pulldown for strobe line in dts

This patch add support to set the internal pulldown via dt property
and allow simplify the board design for the trace from emmc-phy to
the eMMC chipset.
Default to not set the pull-down.

This patch was inspired from the 4.4 tree of the
Rockchip SDK, where it is enabled unconditional.
The patch had been tested with our rk3399 customized board.

Signed-off-by: Chris Ruehl <chris.ruehl@gtsys.com.hk>
Link: https://lore.kernel.org/r/20201129054416.3980-2-chris.ruehl@gtsys.com.hk


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