net: ethernet: ti: cpts: rework locking

Now spinlock is used to synchronize everything which is not required. Add
mutex and use to sync access to PTP interface and PTP worker and use
spinlock only to sync FIFO/events processing.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>