octeontx2-af: Fix ptp timestamp conversion when using external clock

commit 9e2106292bfc709a878941db8e9232f4dc6660ca from
git@git.assembla.com:cavium/WindRiver.linux.git

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

Fixes: 4dca69be61fa ("octeontx2-af: return correct timestamp for cn10k soc")
Change-Id: I1d118bec15355c655cedb86b976617dc2e7e85e7
Signed-off-by: Naveen Mamindlapalli <naveenm@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/99673
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
(cherry picked from commit 98609cc0d26ccf4d5322516bcd1ddd670d60d33c)
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/100784
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>