Add optional reset-gpios pin control. If present, de-assert the
specified reset gpio pin to bring the chip out of reset.

v2:
- Specify that reset signal to PCA953x chip is active low, in
  binding doc.
- reorder includes in gpio-pca953x.c.
- remove dev_err() on devm_gpiod_get_optional() error return.

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: linux-gpio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>