drivers/timer/cavs_timer: Fix race in k_cycle_get_64()

In commit 918a574c ("clock: add k_cycle_get_64") this driver was
augmented with a count64() method to get a 64 bit cycle output from
the two-32-bit-word device registers.

Unfortunately it appeared to be trying to use a spinlock around the
two (low/high) reads to protect against overflow.  But that doesn't
work: spinlocks protect against other CPU code using the same
spinlock, not against a hardware counter that is incrementing in real
time!

Thankfully there was already a count() routine in place that does a
detect-overflow-and-retry loop to solve this.  Use that.

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