arch_timer: add workaround for arm erratum 858921

If CNTVCT or CNTPCT are read while they are updated with the lower
32bits rolled-over, it is possible to read a wrong value(both AArch32 or
AArch64). So if the low 32 bits are set, we need to reread the counter.

Change-Id: Ifd9720c8ff6d0622610b8f53d029635b3130280a
Signed-off-by: Cong Zhang <congzhan@codeaurora.org>