serial: 8250_omap: Add omap8250 earlycon

Add DT earlycon for 8250_omap driver. This boot console is included
for kernels built with CONFIG_SERIAL_EARLYCON=y, CONFIG_OF=y,
CONFIG_SERIAL_8250_OMAP=y, and CONFIG_OF_EARLY_FLATTREE=y.

This boot console is enabled with the command line option "earlycon"
(without "=<name>...") when the DT 'stdout-path' property matches a
compatible uart. For example,

/ {
   chosen {
        stdout-path = "serial0:115200";
   };

   ....

   aliases {
        serial0 = &uart0;
   };

   ....

   ocp : ocp {
        uart0 : serial@44e09000 {
             compatible = "ti,omap3-uart";
        }
   };
};
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_early.c

@@ -149,3 +149,24 @@ EARLYCON_DECLARE(uart8250, early_serial8250_setup);
 EARLYCON_DECLARE(uart, early_serial8250_setup);
 OF_EARLYCON_DECLARE(ns16550, "ns16550", early_serial8250_setup);
 OF_EARLYCON_DECLARE(ns16550a, "ns16550a", early_serial8250_setup);
+
+#ifdef CONFIG_SERIAL_8250_OMAP
+
+static int __init early_omap8250_setup(struct earlycon_device *device,
+				       const char *options)
+{
+	struct uart_port *port = &device->port;
+
+	if (!(device->port.membase || device->port.iobase))
+		return -ENODEV;
+
+	port->regshift = 2;
+	device->con->write = early_serial8250_write;
+	return 0;
+}
+
+OF_EARLYCON_DECLARE(omap8250, "ti,omap2-uart", early_omap8250_setup);
+OF_EARLYCON_DECLARE(omap8250, "ti,omap3-uart", early_omap8250_setup);
+OF_EARLYCON_DECLARE(omap8250, "ti,omap4-uart", early_omap8250_setup);
+
+#endif