devicetree: bindings: ARM: Use "uV" for micro-volt

The binding documentation uses both "uVolt" and "uV" for micro-volt.
Improve consistency by settling on "uV".

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