ARM: dts: imx6sll: Enable SNVS power key according to board design

The SNVS power key 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.

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