ARM: dts: imx: update snvs-poweroff mask

Make the syscon-poweroff driver change only the shutdown bits on the
SNVS_LP control register.

This change fixes an issue with the RTC wakeup alarm, that was
previously disabled during power off.

Signed-off-by: Guy Shapiro <guy.shapiro@mobi-wize.com>
Reviewed-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>