Sorry for the delayed response.
I have been talking to our Power module experts.
Clock module has two tick modes: Periodic and Dynamic. Periodic mode interrupts with each tick whereas Dynamic is a tick suppression mode which reduces the number of timer interrupts to minimum required. You can read more about it here.
So, in short we need to change the Clock to Dynamic mode which will allow the CPU to go sleep mode. In your .cfg file, you can change the Clock mode like the following:
Clock.tickMode = Clock.TickMode_DYNAMIC;
Vikram