x86: Use ePAPR defined properties for x86-uart

Use ePAPR defined properties for x86-uart: clock-frequency and
current-speed. Assign the value of clock-frequency in device tree
to plat->clock of x86-uart instead of using hardcoded number.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

arch/x86/dts/serial.dtsi

@@ -3,8 +3,7 @@
 		compatible = "x86-uart";
 		reg = <0x3f8 8>;
 		reg-shift = <0>;
-		io-mapped = <1>;
-		multiplier = <1>;
-		baudrate = <115200>;
+		clock-frequency = <1843200>;
+		current-speed = <115200>;
 	};
 };

drivers/serial/serial_x86.c

@@ -6,9 +6,12 @@
 
 #include <common.h>
 #include <dm.h>
+#include <fdtdec.h>
 #include <ns16550.h>
 #include <serial.h>
 
+DECLARE_GLOBAL_DATA_PTR;
+
 static const struct udevice_id x86_serial_ids[] = {
 	{ .compatible = "x86-uart" },
 	{ }
@@ -22,10 +25,13 @@ static int x86_serial_ofdata_to_platdata(struct udevice *dev)
 	ret = ns16550_serial_ofdata_to_platdata(dev);
 	if (ret)
 		return ret;
-	plat->clock = 1843200;
+
+	plat->clock = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+				     "clock-frequency", 1843200);
 
 	return 0;
 }
+
 U_BOOT_DRIVER(serial_ns16550) = {
 	.name	= "serial_x86",
 	.id	= UCLASS_SERIAL,