avr: mutex: splitted mutex and spinlock file.

Splitted the mutex and spinlock header in the AVR core to make the
respecitive headers in include/etaos (<etaos/mutex.h> and
<etaos/spinlock.h>) a little bit easier to understand.

This patch is only for clarification and maintainability.

Signed-off-by: Michel Megens <dev@michelmegens.net>