ARM: dove: add si5351 clock driver to CuBox DT

This patch adds the device tree node for si5351 clock generator
and the corresponding oscillator connected to it. It also limits
i2c frequency to 100kHz as there are bus locks reported on higher
frequencies.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>