dt-bindings: Use lower case hex in unit-addresses

DT unit addresses should be lower case hex. Fix all the
binding examples.

Converted with the following command from Krzysztof Kozlowski:

sed -e 's/@\([a-fA-F0-9_-]*\) {/@\L\1 {/' -i $(find Documentation/devicetree/bindings -name '*.txt')

Signed-off-by: Rob Herring <robh@kernel.org>