configure RTC PAD21
| FUN_IE | input enable in work mode |
| SLP_OE | output enable in sleep mode |
| SLP_IE | input enable in sleep mode |
| SLP_SEL | 1: enable sleep mode during sleep,0: no sleep mode |
| FUN_SEL | function sel |
| MUX_SEL | 1: use RTC GPIO,0: use digital GPIO |
| RUE | RUE |
| RDE | RDE |
| DRV | DRV |