ARM: dts: imx6q-bx50v3: Add gpio power off support

bx50v3 boards can be powered off via GPIO, this patch specifies the GPIO to
be used with the gpio-poweroff driver.

Signed-off-by: Ken Lin <ken.lin@advantech.com.tw>
Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>