gpio: pca953x: define masks for addressing common and extended registers

These mask bits are to be used to map the extended register
addresses (which are defined for an unsupported 8-bit pcal chip)
to 16 and 24 bit chips (pcal6524).

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>