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

The compatible strings for "fsl,imx8m*-snvs-lpgpr" always contain
the fallback "fsl,imx7d-snvs-lpgpr" compatible in DTs too, since
the fallback compatible is what the driver matches on, this way:
  compatible = "fsl,imx8mm-snvs-lpgpr", "fsl,imx7d-snvs-lpgpr"
The older "fsl,imx7d-snvs-lpgpr" and "fsl,imx6*-snvs-lpgpr" used
only that single compatible string.

Document both options in the binding document.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>