MPC83xx SerDes controller devices

MPC83xx SoCs contain a built-in SerDes controller that determines which
protocols (SATA, PCI Express, SGMII, ...) are used on the system's serdes lines
and how the lines are configured.

Required properties:
- compatible: must be "fsl,mpc83xx-serdes"
- reg: must point to the serdes controller's register map
- proto: selects for which protocol the serdes lines are configured. One of
  "sata", "pex", "pex-x2", "sgmii"
- serdes-clk: determines the frequency the serdes lines are configured for. One
  of 100, 125, 150.
- vdd: determines whether 1.0V core VDD is used or not

Example:

SERDES: serdes@e3000 {
	reg = <0xe3000 0x200>;
	compatible = "fsl,mpc83xx-serdes";
	proto = "pex";
	serdes-clk = <100>;
	vdd;
};