mlxsw: reg: Use helper macro to define registers

Save some code and also prepare to easily carry name in string form.
Signed-off-by: NJiri Pirko <jiri@mellanox.com>
Acked-by: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>