power: supply: wm97xx_battery: Convert to GPIO descriptor

This converts the WM97xx driver to use a GPIO descriptor
instead of passing a GPIO number thru platform data.

Like everything else in the driver, use a simple local
variable for the descriptor, it can only ever appear in
one instance anyway so it should not hurt.

After converting the driver I noticed that none of the
boardfiles actually define a meaningful GPIO line for
this, but hey, it is converted.

Cc: Haojian Zhuang <haojian.zhuang@gmail.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>