ARM: dts: dm8168-evm: add the external reference clock for SATA

This board has an external oscillator supplying the reference clock
signal for SATA. Its rate is fixed at 100Mhz. Add a corresponding
device tree node.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>