selftests: mlxsw: Add devlink_lib_spectrum.sh

This library builds on top of devlink_lib.sh and contains functionality
specific to Spectrum ASICs, e.g., re-partitioning the various KVD
sub-parts.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
[petrm@mellanox.com: Split this out from another patch. Fix line length
in devlink_sp_read_kvd_defaults().]
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>