serial: 8250_uniphier: add earlycon support

This reuses the code of drivers/tty/serial/8250/8250_early.c except

  - Overwrite device->port.iotype and device->port.regshift for
    UPIO_MEM32 because of_setup_earlycon() has set them for UPIO_MEM.

  - Set device->baud to zero to prevent early8250_setup() from
    initializing the divisor register because port->uartclk does not
    match the frequency expected by this hardware.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_uniphier.c

@@ -13,6 +13,7 @@
  */
 
 #include <linux/clk.h>
+#include <linux/console.h>
 #include <linux/io.h>
 #include <linux/module.h>
 #include <linux/of.h>
@@ -34,6 +35,29 @@ struct uniphier8250_priv {
 	spinlock_t atomic_write_lock;
 };
 
+#ifdef CONFIG_SERIAL_8250_CONSOLE
+static int __init uniphier_early_console_setup(struct earlycon_device *device,
+					       const char *options)
+{
+	if (!device->port.membase)
+		return -ENODEV;
+
+	/* This hardware always expects MMIO32 register interface. */
+	device->port.iotype = UPIO_MEM32;
+	device->port.regshift = 2;
+
+	/*
+	 * Do not touch the divisor register in early_serial8250_setup();
+	 * we assume it has been initialized by a boot loader.
+	 */
+	device->baud = 0;
+
+	return early_serial8250_setup(device, options);
+}
+OF_EARLYCON_DECLARE(uniphier, "socionext,uniphier-uart",
+		    uniphier_early_console_setup);
+#endif
+
 /*
  * The register map is slightly different from that of 8250.
  * IO callbacks must be overridden for correct access to FCR, LCR, and MCR.