pci: Add Rockchip PCIe PHY controller driver

Yes, it is possible to have a dedicated UCLASS PHY driver
for this Rockchip PCIe PHY but there are some issues on
Generic PHY framework to support the same.

The Generic PHY framework is unable to get the PHY if
the PHY parent is of a different uclass.

Say if we try to get the PCIe PHY then the phy-uclass
will look for PHY in the first instance if it is not
in the root node it will try to probe the parent by
assuming that the actual PHY is inside the parent PHY
of UCLASS_PHY. But, in rk3399 hardware representation
PHY like emmc, usb and pcie are part of syscon which
is completely a different of UCLASS_SYSCON.

Example:

grf: syscon@ff770000 {
   compatible = "rockchip,rk3399-grf", "syscon", "simple-mfd";
   reg = <0x0 0xff770000 0x0 0x10000>;
   #address-cells = <1>;
   #size-cells = <1>;

   pcie_phy: pcie-phy {
       compatible = "rockchip,rk3399-pcie-phy";
       clocks = <&cru SCLK_PCIEPHY_REF>;
       clock-names = "refclk";
       #phy-cells = <1>;
       resets = <&cru SRST_PCIEPHY>;
       drive-impedance-ohm = <50>;
       reset-names = "phy";
       status = "disabled";
   };
};

Due to this limitation, this patch adds a separate PHY
driver for Rockchip PCIe. This might be removed in future
once Generic PHY supports this limitation.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Suniel Mahesh <sunil@amarulasolutions.com> #roc-rk3399-pc
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>