tty: serial: 8250: allow to use custom DMA implementation

The OMAP has a few corner cases where it needs a share of kindness of
affection to do the right thing. Heikki Krogerus suggested that instead
adding the quirks into the default DMA implementation, OMAP could get
its own copy of the function. And Alan suggested the same thing so here
we go.

This patch provides callbacks for custom TX/RX DMA implementation. If
there are not setup / used, then the default (current) implementation is
used.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250.h

@@ -16,6 +16,9 @@
 #include <linux/dmaengine.h>
 
 struct uart_8250_dma {
+	int (*tx_dma)(struct uart_8250_port *p);
+	int (*rx_dma)(struct uart_8250_port *p, unsigned int iir);
+
 	/* Filter function */
 	dma_filter_fn		fn;