net: phylink: add legacy_pre_march2020 indicator

Add a boolean to phylink_config to indicate whether a driver has not
been updated for the changes in commit 7cceb599 ("net: phylink:
avoid mac_config calls"), and thus are reliant on the old behaviour.

We were currently keying the phylink behaviour on the presence of a
PCS, but this is sub-optimal for modern drivers that may not have a
PCS.

This commit merely introduces the new flag, but does not add any use,
since we need all legacy drivers to set this flag before it can be
used. Once these legacy drivers have been updated, we can remove this
flag.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>