ARM: dts: imx25: Pass I2C clock-names property

Per i2c-imx.yaml, 'clock-names' is a required property.

Pass it to fix the following dt-schema warning:

imx25-pdk.dtb: i2c@43f80000: clock-names:0: 'ipg' was expected
	from schema $id: http://devicetree.org/schemas/i2c/i2c-imx.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>