phy/micrel: Add support for KSZ8031

Micrel PHY KSZ8031 is similar to KSZ8021 and also requires the special
initialization of "Operation Mode Strap Override" in reg 0x16
introduced in 212ea99a (phy/micrel: Implement support for KSZ8021).

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>