OMAP3: add comments for erratas i540 and i478 workarounds

Add comments and IDs for the following erratas:
- i540: MPU cannot exit from Standby,
- i478: Unexpected Cold-Reset is generated when device is coming
   back from OFF mode

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>