ARM: dts: imx6sll: Enable SNVS poweroff according to board design

The SNVS poweroff depends on board design, by default it should
be disabled in SoC DT and ONLY be enabled on board DT if it is
wired up to external PMIC.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>