gpiobus: Make gpiobus_read_ivar extern

This allows acpi_gpiobus to override the method and fall back to the
generic gpiobus_read_ivar function if needed.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D47250

(cherry picked from commit bc0d10d01c3c69004d600e14d53cd0dceffe4b09)