serial_core: Add wake_peer uart operation

Add wake_peer which is called before starting UART TX. The idea here
is to provide a mechanism where we can wakeup our peer before sending
data.

Change-Id: I42e0779b635f64ca99184b45d5b028de80197491
Signed-off-by: San Mehat <san@google.com>

drivers/tty/serial/serial_core.c

@@ -89,6 +89,9 @@ static void __uart_start(struct tty_struct *tty)
 	struct uart_state *state = tty->driver_data;
 	struct uart_port *port = state->uart_port;
 
+	if (port->ops->wake_peer)
+		port->ops->wake_peer(port);
+
 	if (!uart_circ_empty(&state->xmit) && state->xmit.buf &&
 	    !tty->stopped && !tty->hw_stopped)
 		port->ops->start_tx(port);

include/linux/serial_core.h

@@ -66,6 +66,7 @@ struct uart_ops {
 	void		(*set_ldisc)(struct uart_port *, int new);
 	void		(*pm)(struct uart_port *, unsigned int state,
 			      unsigned int oldstate);
+	void		(*wake_peer)(struct uart_port *);
 
 	/*
 	 * Return a string describing the type of the port