ARM: OMAP2+: timer: add probe for clocksources

A few platforms are currently missing clocksource_probe() completely
in their time_init functionality. On OMAP3430 for example, this is
causing cpuidle to be pretty much dead, as the counter32k is not
going to be registered and instead a gptimer is used as a clocksource.
This will tick in periodic mode, preventing any deeper idle states.

While here, also drop one unnecessary check for populated DT before
existing clocksource_probe() call.

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