power: supply: gpio-charger: Make gpios optional

While strongly recommended, not all devices have a gpio to
detect if the charger is connected. This moves the 'gpios'
from required to optional section.

This also modifies error handling for the GPIO a bit: We
no longer fallback to pdata, if a GPIO is specified using
GPIO descriptor tables. This is a bit cleaner and does
not have any real impact: There are only two mainline pdata
users (arm/mach-sa1100/collie.c, arm/mach-pxa/tosa.c) and
none of them specify the GPIO via gpiod descriptor tables.
Once both have been converted the driver's support for
specifying GPIOs numbers in pdata will be dropped.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>