phy/micrel: add ability to support 50MHz RMII clock on KZS8051RNL

Platform code can now set the MICREL_PHY_50MHZ_CLK bit of dev_flags in a fixup
routine (registered with phy_register_fixup_for_uid()), to make the KZS8051RNL
PHY work with 50MHz RMII reference clock.

Cc: David J. Choi <david.choi@micrel.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>