arm64: dts: ls1028a: support external trigger timestamp fifo of PTP timer

There is an external trigger timestamp fifo for PTP timer
of LS1028A. Add property fsl,extts-fifo for that.

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