gpioaei: Add support for ACPI _EVT method

This method is often used to process GPIO "Power on" button press on
modern x86 laptops with S0ix sleep mode.

Tested with patch from https://reviews.freebsd.org/D26407

Sponsored by: Future Crew LLC
MFC after: 2 month

(cherry picked from commit adc2c156acb50bac6e8ec66b72cadc8910ce4975)