nvmem: prepare basics for FRAM support

Added enum and string for FRAM (ferroelectric RAM) to expose it as file
named "fram".
Added documentation of sysfs file.

Signed-off-by: Jiri Prchal <jiri.prchal@aksignal.cz>
Link: https://lore.kernel.org/r/20210611094601.95131-2-jiri.prchal@aksignal.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>