Hide unused code under #ifdef notyet (in one case the only caller is under
that same ifdef), or if it is arm (not arm64) specific code under the
__arm__ ifdef to not yield -Wunused-function warnings during the arm64
kernel compile.
MFC after: 2 weeks
a10_timer is currently use in UP allwinner SoC (A10 and A13).
Those don't have the generic arm timer.
The arm generic timecounter is broken in the A64 SoC, some attempts have
been made to fix the glitch but users still reported some minor ones.
Since the A64 (and all Allwinner SoC) still have this timer controller, rework
the driver so we can use it in any SoC.
Since it doesn't have the 64 bits counter on all SoC, use one of the
generic 32 bits counter as the timecounter source.
PR: 229644
