fsl-imx6: Add SNVS support for i.MX6 boards

SNVS is supported on both i.MX6 and i.MX6UL and is needed
to support shutdown on the board.

Cc: Peter Maydell <peter.maydell@linaro.org> (odd fixer:SABRELITE / i.MX6)
Cc: Jean-Christophe Dubois <jcd@tribudubois.net> (reviewer:SABRELITE / i.MX6)
Cc: qemu-arm@nongnu.org (open list:SABRELITE / i.MX6)
Cc: qemu-devel@nongnu.org (open list:All patches CC here)
Signed-off-by: Vitaly Cheptsov <cheptsov@ispras.ru>
Message-id: 20230515095015.66860-1-cheptsov@ispras.ru
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>