wl1251: move power GPIO handling into the driver

Move the power GPIO handling from the board code into
the driver. This is a dependency for device tree support.

Signed-off-by: Sebastian Reichel <sre@debian.org>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>