gpio: don't misuse pin->flags

The flags member of gpio_pin_t was only meant for pin flags acquired via
the FDT.

Instead, make an accessor so gpioaei can still access the flags and
remove the now unused gpio_pin_get_by_acpi_index.

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>