digital saradc configure register
| SARADC_MEAS_NUM_LIMIT | enable max meas num |
| SARADC_MAX_MEAS_NUM | max conversion number |
| SARADC_SAR1_INV | 1: data to DIG ADC1 CTRL is inverted, otherwise not |
| SARADC_SAR2_INV | 1: data to DIG ADC2 CTRL is inverted, otherwise not |
| SARADC_TIMER_TARGET | to set saradc timer target |
| SARADC_TIMER_EN | to enable saradc timer trigger |