riscv_machine_timer: fix maximum allowed cycles between reports

There are two issues being fixed here:

1) 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.

2) If a timeout with K_TICKS_FOREVER is provided then the comparator is
   set with UINT64_MAX which is bogus. Not only this value doesn't make
   much sense in the context of a running cycle counter, but it also
   opens the possibility for the same cycle diff and/or ticks overflow
   as above.

Fix both of those 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>