How to reduce voltage with no earth?
You can use a resistor or resistor divider, the problem with this method is the voltage will not be regulated and only works with constant loads. A raspberry pi is a variable load: a resistor or resistor divider won't work. 

A voltage regulator is one of the better ways to regulate voltage, so use that. Make sure you read the datasheet and follow the rules on compensation with capacitors. Most regulators need an input or output capacitor (or both). The LED power supply output should be constant (and regulated), if the LED power supply is unregulated it might not work. Also keep in mind that a rasberry pi needs 500mA of current or more, the LED power supply might not be able to source that much current.







simulate this circuit &ndash; Schematic created using CircuitLab