mfd: dt-bindings: twl4030-power: Use the standard property to mark power control

ti,system-power-controller is more or less the standard way of
indicating that the PMIC is the system wide power controller and hence
may be used to switch off the system. Almost ALL TI PMIC drivers and
many Maxim PMIC drivers follow the same style.

So support 'ti,system-power-controller' in addition to the usual
'ti,use_poweroff' to indicate that the PMIC instance has control for
switching off the system.

Signed-off-by: Nishanth Menon <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/twl4030-power.txt

···
 depending on how the external oscillator is wired.
 
 Optional properties:
-- ti,use_poweroff: With this flag, the chip will initiates an ACTIVE-to-OFF or
-		   SLEEP-to-OFF transition when the system poweroffs.
+
+- ti,system-power-controller: This indicates that TWL4030 is the
+  power supply master of the system. With this flag, the chip will
+  initiate an ACTIVE-to-OFF or SLEEP-to-OFF transition when the
+  system poweroffs.
+
+- ti,use_poweroff: Deprecated name for ti,system-power-controller
 
 Example:
 &i2c1 {