dt-bindings: nvmem: convert U-Boot env to a layout

dt-bindings: nvmem: convert U-Boot env to a layout

U-Boot environment variables can be stored in various data sources. MTD
is just one of available options. Refactor DT binding into a layout so
it can be used with UBI volumes and other NVMEM devices.

Link: https://lore.kernel.org/all/20231221173421.13737-1-zajec5@gmail.com/
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20240902142952.71639-4-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>