regulator: gpio: Get enable GPIO using GPIO descriptor

We augment the GPIO regulator to get the *enable* regulator
GPIO line (not the other lines) using a descriptor rather than
a global number.

We then pass this into the regulator core which has been
prepared to hande enable descriptors in a separate patch.

Switch over the two boardfiles using this facility and clean
up so we only pass descriptors around.

Cc: Philipp Zabel <philipp.zabel@gmail.com> # HX4700/Magician maintainer
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>