mlxsw: reg: Add XM Router M Table Register

The XRMT configures the M-Table for the XLT-LPM.

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