dts: Rename compatible arm,arm-timer to arm,armv8-timer

The compatible for the ARMv8 timer should have been arm,armv8-timer and
not arm,arm-timer.  The dts binding file name was correct, just the
compatible was wrong.  Rename dts, binding, and associated code to use
arm,armv8-timer.

Fixes #31946

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>