MIPS: Loongson: Enable devicetree based probing for 8250 ports in defconfig

After commit 87fcfa7b ("MIPS: Loongson64: Add generic dts"),
there already exists the node and property of Loongson CPU UART0
in loongson3-package.dtsi:

cpu_uart0: serial@1fe001e0 {
        compatible = "ns16550a";
        reg = <0 0x1fe001e0 0x8>;
        clock-frequency = <33000000>;
        interrupt-parent = <&liointc>;
        interrupts = <10 IRQ_TYPE_LEVEL_HIGH>;
        no-loopback-test;
};

In order to support for serial console on the Loongson platform,
add CONFIG_SERIAL_OF_PLATFORM=y to loongson3_defconfig.

With this patch, we can see the following boot messages:

[    1.877745] printk: console [ttyS0] disabled
[    1.881979] 1fe001e0.serial: ttyS0 at MMIO 0x1fe001e0 (irq = 16, base_baud = 2062500) is a 16550A
[    1.890838] printk: console [ttyS0] enabled

And also, we can login normally from the serial console.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/configs/loongson3_defconfig

@@ -217,6 +217,7 @@ CONFIG_SERIAL_8250_EXTENDED=y
 CONFIG_SERIAL_8250_MANY_PORTS=y
 CONFIG_SERIAL_8250_SHARE_IRQ=y
 CONFIG_SERIAL_8250_RSA=y
+CONFIG_SERIAL_OF_PLATFORM=y
 CONFIG_HW_RANDOM=y
 CONFIG_RAW_DRIVER=m
 CONFIG_I2C_CHARDEV=y