dt/bindings: add clock-select function property to micrel phy binding

Add "micrel,rmii-reference-clock-select-25-mhz" to Micrel ethernet PHY
binding documentation.

This property is needed to properly describe some revisions of Micrel
PHYs which has the function of this configuration bit inverted so that
setting it enables 25 MHz rather than 50 MHz clock mode.

Note that a clock reference ("rmii-ref") is still needed to actually
select either mode.

Cc: devicetree@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>