net: phy: at803x: fix Kconfig description

The name of the PHY is actually AR803x not AT803x. Additionally, add the
name of the vendor and mention the AR8031 support.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>