mlxsw: reg: Add XM Direct Register

The XMDR allows direct access to the XM device via the switch.

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