devicetree: Prefix and Postfix helpers for enums

Added helper macros to simplify handling of enum-strings defined in the
dt-bindings. With these new macros you can define C-enums and generate
by parsing the devicetree the enum values.

This makes the devicetree simpler to read and the implementation of
device drivers more straight-forward.

Signed-off-by: Matthias Gilch <[email protected]>