net: phy: micrel: add support for clock-mode select to KSZ8081/KSZ8091

Micrel KSZ8081 and KSZ8091 PHYs have the RMII Reference Clock Select
bit, which is used to select 25 or 50 MHz clock mode.

Note that on some revisions of the PHY (e.g. KSZ8081RND) the function of
this bit is inverted so that setting it enables 25 rather than 50 MHz
mode. Add a new device-tree property
"micrel,rmii-reference-clock-select-25-mhz" to describe this.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>