drivers/cavs_timer: Fix multiword race with timer counter

The count register is 64 bits, but we're a 32 bit CPU that can only
read four bytes at a time, so a bit of care is needed to prevent
racing against a wraparound of the low word.  Wrap the low read
between two reads of the high word and make sure it didn't change.

Fixes #31599

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>