mlxsw: reg: Generare register names automatically

Extend the MLXSW_REG_DEFINE macro to store register name in string form.
Use this string later on instead of hard coded string values.

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