net: phy: lxt: Add suspend/resume support to LXT971 and LXT973.

All LXT PHYs implement the standard "power down" bit 11 of
BMCR, so this patch adds support using the generic
genphy_{suspend,resume} functions added by
commit 0f0ca340 ("phy: power management support").

LXT970 is left aside because all registers get cleared upon
"power down" exit.

Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>