dt-bindings: Document grounded ACOKB pin on S2MPS11

Document a new Device Tree property 'samsung,s2mps11-acokb-ground'
indicating that ACOKB pin of S2MPS11 PMIC is connected to the ground
so the PMIC must manually set PWRHOLD bit in CTRL1 register to turn
off the power.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>

Documentation/devicetree/bindings/mfd/s2mps11.txt

@@ -15,6 +15,10 @@ Optional properties:
 - interrupt-parent: Specifies the phandle of the interrupt controller to which
   the interrupts from s2mps11 are delivered to.
 - interrupts: Interrupt specifiers for interrupt sources.
+- samsung,s2mps11-acokb-ground: Indicates that ACOKB pin of S2MPS11 PMIC is
+  connected to the ground so the PMIC must manually set PWRHOLD bit in CTRL1
+  register to turn off the power. Usually the ACOKB is pulled up to VBATT so
+  when PWRHOLD pin goes low, the rising ACOKB will trigger power off.
 
 Optional nodes:
 - clocks: s2mps11, s2mps13 and s5m8767 provide three(AP/CP/BT) buffered 32.768