dt-bindings: ptp: Add device tree binding for IDT ClockMatrix based PTP clock

Add device tree binding doc for the IDT ClockMatrix PTP clock.

Signed-off-by: Vincent Cheng <vincent.cheng.xh@renesas.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>