pinctrl: clarify some dt pinconfig options

pinctrl: clarify some dt pinconfig options

The bias-pull-* options use values > 0 to indicate that the pull should
be activated and optionally also indicate the strength of the pull.
Therefore use an default value of 1 for these options.

Split the low-power-mode option into low-power-enable and -disable.

Update the documentation to describe the param arguments better.

Reported-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
(cherry picked from commit 9ee1f7d266aa1e2bfeb20cb5d4ac299c8e8ef8c7)
Signed-off-by: Mark Brown <broonie@linaro.org>