arm64: dts: imx8mm: Add SNVS LPGPR

Add SNVS LPGPR bindings to MX8M Mini, the LPGPR is used to store
boot counter.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: devicetree@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>