dt-bindings: nvmem: u-boot,env: add MAC's #nvmem-cell-cells

dt-bindings: nvmem: u-boot,env: add MAC's #nvmem-cell-cells

U-Boot's "ethaddr" environment variable is very often used to store
*base* MAC address. It's used as a base for calculating addresses for
multiple interfaces. It's done by adding proper values. Actual offsets
are picked by manufacturers and vary across devices.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-34-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>