drivers/rpmsg_port_uart: switch the uart IO to GPIO when peer poweroff

Use the TIOCVHANGUP to notify the driver to switch the uart io
to gpio mode to avoid the current leak caused by the uart io because
peer core may already poweroff so the io can be in high voltage
state.

Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

drivers/rpmsg/rpmsg_port_uart.c

@@ -400,6 +400,7 @@ static int rpmsg_port_uart_rx_thread(int argc, FAR char *argv[])
               rpmsgdbg("Received poweroff command\n");
               rpmsg_port_drop_packets(&rpuart->port, RPMSG_PORT_DROP_TXQ);
               rpmsg_port_unregister(&rpuart->port);
+              DEBUGVERIFY(file_ioctl(&rpuart->file, TIOCVHANGUP, 0) >= 0);
               rpuart->connected = false;
               continue;
             }