Kconfig: Select GENERIC_CLOCKEVENTS_BROADCAST for sdxprairie target

Enable Broadcast timer support for uniprocessor systems. When a CPU
enters power collapse, the system timers are held in reset and the CPU
relies on the broadcast timers to wakeup the CPU up for timer events.

Change-Id: Idc3d77e5a987b0bcbee59913b23325e83a35b3e5
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>