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>