gpio: pca953x: Add support for pca9506

According to the reference manual "The PCA9505 is identical to the
PCA9506 except that it includes 100 kΩ internal pull-up resistors on all
the I/Os." So the pca9506 device can be considered identical to the
pca9505 for the gpio driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>