rtc: omap: Add external clock enabling support

Configure the clock source to external clock if available.
External clock is preferred as it can be ticking during suspend.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>