sunxi: add gpio driver

This patch enables CONFIG_CMD_GPIO for the Allwinner (sunxi) platform as well
as providing the common gpio API (gpio_request/free, direction in/out, get/set
etc).

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ma Haijun <mahaijuns@gmail.com>
Signed-off-by: Oliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: Henrik Nordström <henrik@henriknordstrom.net>
Cc: Tom Cubie <Mr.hipboi@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>