ARM: dts: socfpga: set the i2c frequency

Use 'clock-frequency' binding for the i2c node that will put the I2C driver
into the standard operating mode. 'speed-mode' was not a valid binding for
the I2C driver, remove it.

Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>