arm64: dts: imx8mp: Add SNVS LPGPR

Add SNVS LPGPR bindings to MX8M Plus, the LPGPR is used to store
e.g. boot counter.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>