ARM: dts: logicpd-som-lv: 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: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>