net: mscc: ocelot: get HSIO regmap from syscon

HSIO address space was moved to a syscon, hence we need to get the
regmap of this address space from there and no more from the device
node.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>