OMAP3: clean-up mach specific cpuidle data structures

- sleep_latency and wake_latency are not used, replace them by
  exit_latency which is used by cpuidle. exit_latency simply is
  the sum of sleep_latency and wake_latency,
- replace threshold by target_residency,
- changed the OMAP3 specific cpuidle code accordingly,
- changed the OMAP3 board code accordingly.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>