gpio / ACPI: Rename acpi_gpio_evt_pin to acpi_gpio_event

In order to consolidate _Exx, _Lxx and _EVT to use the same structure make
the structure name to reflect that we are dealing with any event, not just
_EVT.

This is just rename, no functional changes.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>