configure RTC PAD18
| PDAC2_DAC | PDAC2_DAC |
| PDAC2_XPD_DAC | PDAC2_XPD_DAC |
| PDAC2_DAC_XPD_FORCE | 1: use reg_pdac2_xpd_dac to control PDAC2_XPD_DAC,0: use SAR ADC FSM to control PDAC2_XPD_DAC |
| PDAC2_FUN_IE | input enable in work mode |
| PDAC2_SLP_OE | output enable in sleep mode |
| PDAC2_SLP_IE | input enable in sleep mode |
| PDAC2_SLP_SEL | 1: enable sleep mode during sleep,0: no sleep mode |
| PDAC2_FUN_SEL | PDAC1 function sel |
| PDAC2_MUX_SEL | 1: use RTC GPIO,0: use digital GPIO |
| PDAC2_RUE | PDAC2_RUE |
| PDAC2_RDE | PDAC2_RDE |
| PDAC2_DRV | PDAC2_DRV |