octeontx2-af: return correct ptp timestamp for CN10K silicon

The MIO_PTP_TIMESTAMP format has been changed in CN10K silicon
family. The upper 32-bits represents seconds and lower 32-bits
represents nanoseconds. This patch returns nanosecond timestamp
to NIX PF driver.

Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Signed-off-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>