mlxsw: core_linecards: Implement line card device flashing

Implement flash_update() devlink op for the line card devlink instance
to allow user to update line card gearbox FW using MDDT register
and mlxfw.

Example:
$ devlink dev flash auxiliary/mlxsw_core.lc.0 file mellanox/fw-AGB-rel-19_2010_1312-022-EVB.mfa2

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>