dt-bindings: arm: Clean up CPU binding examples

Following commit 31af04cd

 ("arm64: dts: Remove inconsistent use of
'arm,armv8' compatible string"), clean up these binding examples in case
anyone is tempted to copy them.

CC: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>