powerpc/of_serial: Add missing ns16550a id

Many boards have a bug-free ns16550 compatible serial port, which we should
register as PORT_16550A. This introduces a new value "ns16550a" for the
compatible property of of_serial to let a firmware choose that model instead
of using the crippled PORT_16550 mode.
Reported-by: Alon Ziv <alonz@nolaviz.org>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

drivers/serial/of_serial.c

@@ -161,6 +161,7 @@ static int of_platform_serial_remove(struct of_device *ofdev)
 static struct of_device_id __devinitdata of_platform_serial_table[] = {
 	{ .type = "serial", .compatible = "ns8250",   .data = (void *)PORT_8250, },
 	{ .type = "serial", .compatible = "ns16450",  .data = (void *)PORT_16450, },
+	{ .type = "serial", .compatible = "ns16550a", .data = (void *)PORT_16550A, },
 	{ .type = "serial", .compatible = "ns16550",  .data = (void *)PORT_16550, },
 	{ .type = "serial", .compatible = "ns16750",  .data = (void *)PORT_16750, },
 	{ .type = "serial", .compatible = "ns16850",  .data = (void *)PORT_16850, },