mlxsw: reg: Add Router LPM Cache Enable Register

The RLPMCE allows disabling the LPM cache. Can be changed on the fly.

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