gpio: loongson1: Introduce ls1x_gpio_chip struct

This patch introduces and allocates ls1x_gpio_chip struct containing
gpio_chip and reg_base to avoid global gpio_reg_base.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>