gpiolib: Update documentation of struct acpi_gpio_info

It seems the code had been changed, but description left untouched.

Update description of the struct acpi_gpio_info and relative comments
accordingly.

Fixes: commit 52044723 ("ACPI / gpio: Add irq_type when a GPIO is used as an interrupt")
Cc: Christophe RICARD <christophe.ricard@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>