net: mscc: ocelot: convert QSYS_SWITCH_PORT_MODE and SYS_PORT_MODE to regfields

Currently Felix and Ocelot share the same bit layout in these per-port
registers, but Seville does not. So we need reg_fields for that.

Actually since these are per-port registers, we need to also specify the
number of ports, and register size per port, and use the regmap API for
multiple ports.

There's a more subtle point to be made about the other 2 register
fields:
- QSYS_SWITCH_PORT_MODE_SCH_NEXT_CFG
- QSYS_SWITCH_PORT_MODE_INGRESS_DROP_MODE
which we are not writing any longer, for 2 reasons:
- Using the previous API (ocelot_write_rix), we were only writing 1 for
  Felix and Ocelot, which was their hardware-default value, and which
  there wasn't any intention in changing.
- In the case of SCH_NEXT_CFG, in fact Seville does not have this
  register field at all, and therefore, if we want to have common code
  we would be required to not write to it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>