acpi_gpiobus: Narrow scope of NOT_YET

GPIO interrupts work just fine and will be used shortly. We still
do not support GPIO_INTR_SHAREABLE however, so leave that within
the NOT_YET scope.

Reviwed by: andrew
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D47251