dt-bindings: ptp: Add virtual ptp memory entry

* Qcom virtual gPTP Data based shared memory IPC

Required properties:

- compatible: should contain the core compatibility string

              and the SoC compatibility string. The SoC

              compatibility string is to handle SoC specific

              hardware differences.

              Core compatibility string:

                 "qcom,ptp_virtual"

- reg: address and length of the shared memory for gPTP data

Example:

ptp_virtual {

	compatible = "qcom,ptp_virtual";

	reg = <0xeb600000 0x1000>;

};