ARM: dts: omap5: add fref_xtal_ck support

The clock is directly sourced from sys_clkin, and provides an external
output clock for (typically) TWL6040 chip.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>