regulator: gpio: fix parsing of gpio list

The list of gpios is defined as optional but the code was
failing to properly handle the case of no gpios, and also
failing to check for errors reading the entry from the
devicetree.

This patch fixes the handling of optional gpios - this is a
useful feature enabling the gpio-regulator to be used as a
dummy variable voltage regulator without having to assign any
real GPIO lines.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>