net: phylink: provide phylink_mac_implements_lpi()

Provide a helper to determine whether the MAC operations structure
implements the LPI operations, which will be used by both phylink and
DSA.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1thR9g-003vX6-4s@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>