ARM: dts: NSP: Fix Ethernet switch SGMII register name

The register name should be "sgmii_config", not "sgmii", this is not a
functional change since no code is currently looking for that register
by name (or at all).

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>