gpio: pca953x: Zap ad-hoc reg_output cache

Replace the ad-hoc reg_output output register caching with generic
regcache cache. Drop pca953x_write_single() which is no longer used.
This reduces code duplication.

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>