mfd: dt-bindings: Document pulled down WRSTBI pin on S2MPS1X

Document a new Device Tree property 'samsung,s2mps11-wrstbi-ground'
indicating that WRSTBI pin of S2MPS13 PMIC is pulled down so
corresponding buck warm reset function should be disabled.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>