gpiolib: acpi: Fix references in kernel doc and amend

This patch does the following bunch of changes:
 - append () to the functions for reference
 - convert gpiochip(s) -> GPIO chip(s)
 - add a note about returned error code type [acpi_find_gpio()]
 - move long summary to a description [acpi_gpio_count()]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>