dt-bindings: property-units: Sanitize unit naming

Make the naming of units consistent with common practices:
  - Do not capitalize the first character of units ("Celsius" is
    special, as it is not the unit name, but a reference to its
    proposer),
  - Do not use plural for units,
  - Do not abbreviate "ampere",
  - Concatenate prefixes and units (no spaces or hyphens),
  - Separate units by spaces not hyphens,
  - "milli" applies to "degree", not to "Celsius".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>