add overlay for enabling serial1 / uart1 on rk3328
On Rock64 the uart is routed to pins on the "Pi-2" header, so it is
potentially useful.
Pin mapping:
| ID | Name | Function |
| 15 | GPIO3_A4 | TX |
| 16 | GPIO3_A5 | RTS |
| 18 | GPIO3_A6 | RX |
| 22 | GPIO3_A7 | CTS |
MFC after: 2 weeks