OMAP3: rework of the ASM sleep code execution paths

- Reworked and simplified the execution paths for better
  readability and to avoid duplication of code,
- Added comments on the entry and exit points and the interaction
  with the ROM code for OFF mode restore,
- Reworked the existing comments for better readability.

Tested on N900 and Beagleboard with full RET and OFF modes,
using cpuidle and suspend.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>