octeontx2-bphy-netdev: Fix ptp timestamp conversion when using external clock

commit d73944e902afb173b2738f2be3e731430e54ac9a from
git@git.assembla.com:cavium/WindRiver.linux.git

This patch fixes ptp timestamp conversion for different variants of
cn10k silicon.

Fixes: 0c135c5a8505 ("octeontx2-bphy-netdev: convert MIO_PTP_TIMESTAMP value to nsecs")
Change-Id: I911db186a7da42ef3dc5f45995327630414ff86d
Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/99674
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
(cherry picked from commit 0b687842a80aa0e824a7f1fef0e757df1ab06742)
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/100787
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>