dt-bindings: nvmem: snvs-lpgpr: Add i.MX8M compatible strings

Add compatible strings for i.MX8M(Q), i.MX8M Mini, i.MX8M Nano, i.MX8M Plus.
All these SoCs have the SNVS LPGPR registers and they are at the same offset
as on i.MX7D.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anson Huang <Anson.Huang@nxp.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Oleksij Rempel <linux@rempel-privat.de>
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: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220407103016.16543-1-marex@denx.de