ARM: dts: dm816x: add fck under timers1/2

Add the functional clock definition for timers1/2. This is needed so that
the clock rate claculations continue to function properly once dm816x
transitions away from hwmod data and towards the clkctrl clocks.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>