video: slimport: Do not access the sp registers after power down

The burst i2c errors sometimes causes a watchdog reset. The i2c errors
are caused by accessing the sp registers after SP is power down. So
do not access the sp register after power down to avoid the watchdog reset.

Change-Id: If64b0bf6f437292f356fa165d4203d522c26cfe6
Signed-off-by: Devin Kim <dojip.kim@lge.com>

Conflicts:
	drivers/video/slimport/slimport.c
Power: Changes the permission to read only for sysfs file
/sys/kernel/wakeup_reasons/last_resume_reason

Also changes the sysfs read callback name so that __ATTR_RO can be used.

Change-Id: I9b625f6cafcb35d207def4157d0fb4a9964104de
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>