ARM: dts: am57xx-beagle-x15: Move clkout2 source selection to codec node

The assigned-clock* needs to be in the root of the device's node. If it is
in the sub-node the CCF will ignore it.
Since the clkout2 is used by the codec as MCLK, move the clock parent
selection to that node.

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