nvmem: brcm_nvram: parse NVRAM content into NVMEM cells

NVRAM consist of header and NUL separated key-value pairs. Parse it and
create NVMEM cell for every key-value entry.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220225175822.8293-3-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>