Add pldmfw library for PLDM firmware update

The pldmfw library is used to implement common logic needed to flash
devices based on firmware files using the format described by the PLDM
for Firmware Update standard.

This library consists of logic to parse the PLDM file format from
a firmware file object, as well as common logic for sending the relevant
PLDM header data to the device firmware.

A simple ops table is provided so that device drivers can implement
device specific hardware interactions while keeping the common logic to
the pldmfw library.

This library will be used by the Intel ice networking driver as part of
implementing device flash update via devlink. The library aims to be
vendor and device agnostic. For this reason, it has been placed in
lib/pldmfw, in the hopes that other devices which use the PLDM firmware
file format may benefit from it in the future. However, do note that not
all features defined in the PLDM standard have been implemented.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>