MIPS: dts: mscc: describe the PTP ready interrupt

This patch adds a description of the PTP ready interrupt, which can be
triggered when a PTP timestamp is available on an hardware FIFO.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: davem@davemloft.net
Cc: richardcochran@gmail.com
Cc: alexandre.belloni@bootlin.com
Cc: UNGLinuxDriver@microchip.com
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-mips@vger.kernel.org
Cc: thomas.petazzoni@bootlin.com
Cc: allan.nielsen@microchip.com