ARM: dts: omap5-board-common: Add phandle for mclk clock for twl6040

The xref_xtal clock is used by twl6040 as mclk. It is needed for the HPPLL
internally.

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