gpio: mvebu: Remove initcall-based driver initialization

There's no reason to use an initcall to initialize this driver,
and regular module_platform_driver() can be used instead.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>