drivers: serial: Add USART support for GD32V

Modifying configuration to enable with gd32vf103

- Add usart definition to devicetree.
- Define USART_STAT as alias of USART_STAT0 if not defined it.
- Enable USART if SOC_SERIES_RISCV_GIGADEVICE_GD32VF103.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>