gpio: bd9571mwv: rid of using struct bd9571mwv

To simplify this driver, use dev_get_regmap() and
rid of using struct bd9571mwv.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>