ARM: dts: socfpga: set desired i2c clock on Cyclone5 and Arria5 devkits

The I2C LCD display on the Cyclone5 and Arria5 devkits is only capable of
the standard 100 kHz clock. Set the "clock-frequency" of the I2C node
to be 100000.

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