ARM: dts: imx6ul-var-som: fix warning for boolean property with a value

ARM: dts: imx6ul-var-som: fix warning for boolean property with a value

dmesg warning:

  OF: /soc/bus@2000000/ethernet@20b4000/mdio/ethernet-phy@3: Read of
      boolean property 'micrel,rmii-reference-clock-select-25-mhz' with a
      value.

Using of_property_read_bool() for non-boolean properties is deprecated and
results in a warning during runtime since commit c141ecc3cecd
("of: Warn when of_property_read_bool() is used on non-boolean properties")

micrel,rmii-reference-clock-select-25-mhz is a boolean property and should
not have a value, so remove it.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>