nvmem: brcm_nvram: add .read_post_process() for MACs

1. Parse ASCII MAC format into byte based
2. Calculate relative addresses based on index argument

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Message-ID: <20230611140330.154222-5-srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>