MIPS: mscc: ocelot: make HSIO registers address range a syscon

HSIO contains registers for PLL5 configuration, SerDes/switch port
muxing and a thermal sensor, hence we can't keep it in the switch DT
node.

Acked-by: Paul Burton <paul.burton@mips.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>