mlxsw: reg: Add Router XLT M select Register

The RXLTM configures and selects the M for the XM lookups.

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