arm64: dts: imx8mm-kontron: Set lower limit of VDD_SNVS to 800 mV

According to the datasheet the typical value for VDD_SNVS should be
800 mV, so let's make sure that this is within the range of the
regulator.

Fixes: 8668d8b2

 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Cc: stable@vger.kernel.org
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>