ARM: dts: omap3-beagle-xm: Add fixed 26MHz clock as fck for twl

The board uses 26MHz oscillator for the twl4030 HFCLK.
This way we will not depend on the bootloader to configure the
CFG_BOOT:HFCLK_FREQ

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>