net/phy: micrel: Add clock support for KSZ8021/KSZ8031

The KSZ8021 and KSZ8031 support RMII reference input clocks of 25MHz
and 50MHz. Both PHYs differ in the default frequency they expect
after reset. If this differs from the actual input clock, then
register 0x1f bit 7 must be changed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>