Documentation: pinctrl: palmas: Add ti,palmas-powerhold-override property definition

GPIO7 is configured in POWERHOLD mode which has higher priority
over DEV_ON bit and keeps the PMIC supplies on even after the DEV_ON
bit is turned off. This property enables driver to over ride the
POWERHOLD value to GPIO7 so as to turn off the PMIC in power off
scenarios.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>