mlxsw: spectrum: Change KVD linear parts from list to array

The parts info is array. The parts copy this info array, yet they are a
list. So make the indexing according to the id and change the list of
parts into array of parts. This helps to eliminate lookups and
constructs like mlxsw_sp_kvdl_part_update() (took me some non-trivial
time to figure out what is going on there).
Alongside with that, introduce a helper macro to define the parts infos.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>