scsi: ufs: sysfs: reworking of the rpm_lvl and spm_lvl entries

Read from these files will return the integer value of the chosen power
management level now. Separate entries were added to show the target UFS
device and UIC link states. The description of the possible power
managements levels was added to the ABI file. The on-write behaviour of
these entries wasn't changed.

[mkp: typo]

Signed-off-by: Stanislav Nijnikov <stanislav.nijnikov@wdc.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>