nvmem: core: add nvmem cell post processing callback

Some NVMEM providers have certain nvmem cells encoded, which requires
post processing before actually using it.

For example mac-address is stored in either in ascii or delimited or reverse-order.

Having a post-process callback hook to provider drivers would enable them to
do this vendor specific post processing before nvmem consumers see it.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>