mlxsw: reg: Add Router General Configuration Register

Add the Router General Configuration Register (RGCR), which allows us to
enable the router in the device and configure its various parameters.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>