gpio: gpio-aspeed-sgpio: Add set_config function

AST SoC supports *retain pin state* function when wdt reset.
The patch adds set_config function for handling sgpio reset tolerance
register.

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>