Add device tree binding doc for the PTP clock based on IDT 82P33
Synchronization Management Unit (SMU).

Changes since v1:
 - As suggested by Rob Herring:
   1. Drop reg description for i2c
   2. Replace i2c@1 with i2c
   3. Add addtionalProperties: false
Signed-off-by: Min Li <min.li.xe@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>