devicetree: bindings: add bindings for ahci-da850

Device tree binding for the TI DA850 AHCI SATA Controller

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Required properties:

  - compatible: must be "ti,da850-ahci"

  - reg: physical base addresses and sizes of the two register regions

         used by the controller: the register map as defined by the

         AHCI 1.1 standard and the Power Down Control Register (PWRDN)

         for enabling/disabling the SATA clock receiver

  - interrupts: interrupt specifier (refer to the interrupt binding)

Example:

	sata: sata@218000 {

		compatible = "ti,da850-ahci";

		reg = <0x218000 0x2000>, <0x22c018 0x4>;

		interrupts = <67>;

	};