Documentation: tps65912: Add DT bindings for the TPS65912 PMIC

The TPS65912 PMIC contains several regulators and a GPIO controller.
Add bindings for the TPS65912 PMIC.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>