RDA5981X: Add SERIAL_FC

According to comment in the implementation this target supports the
hardware flow control on UART1 peripheral.
This patch fixes build errors after adding DEVICE_SERIAL_FC guards to
hal/serial_api.h.

Author: Filip Jagodzinski

targets/targets.json

@@ -8556,6 +8556,7 @@
             "INTERRUPTIN",
             "EMAC",
             "SERIAL",
+            "SERIAL_FC",
             "STDIO_MESSAGES",
             "PWMOUT",
             "SPI",