gpio: pca953x: Perform basic regmap conversion

Convert the driver to use regmap to access the chips. Due to the convoluted
register mapping scheme, implement read/write/volatile check functions that
untangle the mess and perform check accordingly. This patch does not zap the
internal register cache of the PCA953x driver, nor does it push the regmap
access down into the gpiochip accessors to simplify the review. All that is
in subsequent patches.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>