arm64: dts: fsl: add fsl,extts-fifo property for fman ptp

The 1588 timer supports external trigger timestamp FIFO on
FMan on QorIQ ARM platforms.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>