add 8Mhz mode to revert to internal slow clock
add 8Mhz mode to revert to internal slow clock

jeelabs/embello,smurfix/embello,smurfix/embello,smurfix/embello,jeelabs/embello,tve/embello,tve/embello,jeelabs/embello,jeelabs/embello,tve/embello,jeelabs/embello