power: supply: max1720x: add read support for nvmem

ModelGauge m5 and device configuration values are stored in nonvolatile
memory to prevent data loss if the IC loses power. Add read support for
the nonvolatile memory on MAX1720X devices.
Signed-off-by: Dimitri Fedrau <dima.fedrau@gmail.com>
Link: https://lore.kernel.org/r/20240903063526.222890-1-dima.fedrau@gmail.comSigned-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>