gpio: pca953x: Add TI TCA9539 support

The TCA9539 is almost identical to the PCA9555 and software-compatible
with this driver. It exposes 16 general purpose I/O pins in two 8-bit
configurations.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>