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>