nvmem: u-boot-env: post-process "ethaddr" env variable

nvmem: u-boot-env: post-process "ethaddr" env variable

U-Boot environment variables are stored in ASCII format so "ethaddr"
requires parsing into binary to make it work with Ethernet interfaces.

This includes support for indexes to support #nvmem-cell-cells = <1>.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20230404172148.82422-36-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>