OSTCCR_PRESCALE2_MASK
prescale = FIELD_GET(OSTCCR_PRESCALE2_MASK, prescale);
val &= ~OSTCCR_PRESCALE2_MASK;
val |= FIELD_PREP(OSTCCR_PRESCALE2_MASK, prescale);
val &= ~(OSTCCR_PRESCALE1_MASK | OSTCCR_PRESCALE2_MASK);