pinctrl: sx150x: make struct sx150x_regmap_config static

The structure sx150x_regmap_config is local to the source and does not
need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'sx150x_regmap_config' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>