phy: marvell: add link partner advertised modes

Read the standard link partner advertisment registers and store it in
phydev->lp_advertising, so ethtool can report this information to
userspace via ethtool.  Zero it as per genphy if autonegotiation is
disabled.  Tested with a Marvell 88E1512 PHY.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>