gpio: pca953x: Expand comment for "reset" GPIO in ACPI case

GPIO ACPI library is going to be stricter about resources, thus, expand
comment regarding "reset" GPIO resource in this driver to clarify its
usage in ACPI case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>