dt-bindings: phy-rockchip-pcie: Convert to per-lane PHY model

Deprecate the legacy Rockchip PCIe PHY and encourage users to use per-lane
PHY mode by setting #phy-cells to 1.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>