net: mscc: ocelot: convert port registers to regmap

At the moment, there are some minimal register differences between
VSC7514 Ocelot and VSC9959 Felix. To be precise, the PCS1G registers are
missing from Felix because it was integrated with an NXP PCS.

But with VSC9953 Seville (not yet introduced), the register differences
are more pronounced.  The MAC registers are located at different offsets
within the DEV_GMII target. So we need to refactor the driver to keep a
regmap even for per-port registers. The callers of the ocelot_port_readl
and ocelot_port_writel were kept unchanged, only the implementation is
now more generic.

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>