selftests: mlxsw: Add devlink KVD resource test

Add a selftest that can be used to perform basic sanity of the devlink
resource API as well as test the behavior of KVD manipulation in the
driver.

This is the first case of a HW-only test - in order to test the devlink
resource a driver capable of exposing resources has to be provided
first.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
[petrm@mellanox.com: Extracted two patches out of this patch. Tweaked
commit message.]
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>