Spinel.h: Sync spinel header file with openthread
Spinel.h: Sync spinel header file with openthread

This syncs `spinel.h` with openthread at commit adba70a3906.

openthread/wpantund,openthread/wpantund,d-bahr/wpantund,aeliot/wpantund,openthread/wpantund,d-bahr/wpantund,aeliot/wpantund,abtink/wpantund,abtink/wpantund,aeliot/wpantund,openthread/wpantund,aeliot/wpantund,abtink/wpantund,abtink/wpantund,d-bahr/wpantund,d-bahr/wpantund