gpio: pca953x: Factor out common code from device_pca95xx_init()

The PCA957x and PCA953x init functions are almost the same, except for
the different register mapping and one extra write to BKEN register in
case of PCA957x. Factor out the common code.

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>