regulator: gpio: Reword the binding document

regulator: gpio: Reword the binding document

Reword the binding document to make it clear how the propeties work
and which properties affect which other properties.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Harald Geyer <harald@ccbib.org>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: linux-renesas-soc@vger.kernel.org
To: devicetree@vger.kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>