ARM: omap3: cpuidle: enable time keeping

The TIME_VALID flag is specified for the different states but
the time residency computation is not done, no tk flag, no time
computation in the idle function.

Set the en_core_tk_irqen flag to activate it.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>