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
Approved by: re (kib)
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D47251

(cherry picked from commit 2d4219919a48aa6d67ae62e2b8859ffe3b035bd2)
(cherry picked from commit 1f6941760746367704c9f1fa730a00b99d4d46b2)