ARM: dts: am43xx-clocks: Add support for CLKOUT2

Add the needed clock nodes for the CLKOUT2 to be usable by boards.

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