ARM: dts: am33xx-clock: Fix ehrpwm tbclk data

tbclk does not need to be a composite clock, we can simply
use gate clock for this purpose.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>