gpioaei: mark the AEI ithread as sleepable

We call an ACPICA function in the interrupt handler which may attempt to
lock a mutex. If this is done while another instance of the handler
already holds the lock, it would cause a kernel panic since sleeping
isn't allowed.

Reviewed-by: Warner Losh <imp@FreeBSD.org>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1667
Signed-off-by: Ahmad Khalifa <ahmadkhalifa570@gmail.com>