scsi: ufs: Add "wb_on" sysfs node to control WB on/off

scsi: ufs: Add "wb_on" sysfs node to control WB on/off

Currently UFS WriteBooster driver uses clock scaling up/down to set WB
on/off. For the platforms which don't support UFSHCD_CAP_CLK_SCALING, WB
will be always on. Provide a sysfs attribute to enable/disable WB during
runtime. Write 1/0 to "wb_on" sysfs node to enable/disable UFS WB.

Link: https://lore.kernel.org/r/20210119163847.20165-2-huobean@gmail.com
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Bean Huo <beanhuo@micron.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>