ARM: dts: meson8b: add the I2C clocks

Add the I2C clocks so the I2C busses can be used. The clock input is not
device specific (the AO I2C bus uses clk81 as input, while the two I2C
busses in CBUS have a separate "CLKID_I2C" gate, provided by the clock
controller.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>