octeontx2-af: Add external ptp input clock

PTP hardware block can be configured to utilize
the external clock. Also the current ptp timestamp
can be captured when external trigger is applied on
a gpio pin. These features are required in scenarios
like connecting a external timing device to the chip
for time synchronization. The timing device provides
the clock and trigger(PPS signal) to the PTP block.
This patch does the following:
1. configures PTP block to use external clock
frequency and timestamp capture on external event.
2. sends PTP_REQ_EXTTS events to kernel ptp phc susbsytem
with captured timestamps
3. aligns PPS edge to adjusted ptp clock in the ptp device
by setting the PPS_THRESH to the reminder of the last
timestamp value captured by external PPS

Signed-off-by: Yi Guo <yig@marvell.com>
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: Sunil Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>