scsi: dt-bindings: ufs: Remove custom property "<name>-fixed-regulator"

Custom device tree property "<name>-fixed-regulator" can be removed
because,

1) It is not used anywhere in tree.

2) All handlings for "<name>-fixed-regulator" have been removed in UFS
   driver.

3) There is no issue if an existed regulator defined
   "<name>-fixed-regulator" property in device tree.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>