ARM: dts: dra7: add fck under timer1

Add the functional clock definition for timer1. This is needed so that
the clock rate calculations continue to function properly once dra7
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>