dt-binding: ptp: Add SoC compatibility strings for dte ptp clock

Add SoC specific compatibility strings to the Broadcom DTE
based PTP clock binding document.

Fixed the document heading and node name.

Fixes: 80d60761

 ("dt-binding: ptp: add bindings document for dte based ptp clock")
Signed-off-by: Arun Parameswaran <arun.parameswaran@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>