dt-binding: ptp: add bindings document for dte based ptp clock

Add device tree binding documentation for the Broadcom DTE
PTP clock driver.
Signed-off-by: NArun Parameswaran <arun.parameswaran@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/ptp/brcm,ptp-dte.txt

+* Broadcom Digital Timing Engine(DTE) based PTP clock driver
+
+Required properties:
+- compatible: should be "brcm,ptp-dte"
+- reg: address and length of the DTE block's NCO registers
+
+Example:
+
+ptp_dte: ptp_dte@180af650 {
+	compatible = "brcm,ptp-dte";
+	reg = <0x180af650 0x10>;
+	status = "okay";
+};