gpio: pca953x: add ->set_config implementation

This commit adds a minimal implementation of the ->set_config() hook,
with support for the PIN_CONFIG_BIAS_PULL_UP and
PIN_CONFIG_BIAS_PULL_DOWN configurations.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>