ARMV7: S5P: rename the member of gpio structure

Typically we declare the name of gpio structure to "gpio",
so it was duplicated around the name. (e.g: gpio->gpio_a)
This patch modified the naming that is removing "gpio_".

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>