arm_arch_timer: fix maximum allowed cycles between reports

The code currently clamps timeout length so not to overflow the computed
cycle difference variable or the sys_clock_announce() argument's range.
But this completely fails to take into account the case where two
successive timeouts with enough time between them will still overflow the
cycle difference and/or the tick count.

Fix this by clamping the actual number of cycles to wait for based on
the previous report occurrence rather than clamping the timeout ticks.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>