net: phylink: provide phylink_pcs_config() and phylink_pcs_link_up()

commit  bf134ce998706ba7ba1476d460461b439f72c395 from
https://github.com/nxp-imx/linux-imx lf-6.1.y

Add two helper functions for calling PCS methods. phylink_pcs_config()
allows us to handle PCS configuration specifics in one location, rather
than the two call sites. phylink_pcs_link_up() gives us consistency.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1q1TzK-007Exd-Rs@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit ae4899bb

)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>