MIPS: Loongson: Make platform serial setup always built-in.

If SERIAL_8250 is compiled as a module, the platform specific setup
for Loongson will be a module too, and it will not work very well.
At least on Loongson 3 it will trigger a build failure,
since loongson_sysconf is not exported to modules.

Fix by making the platform specific serial code always built-in.
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Reported-by: Ralf Baechle <ralf@linux-mips.org>
Cc: stable@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Markos Chandras <Markos.Chandras@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/8533/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/loongson/common/Makefile

@@ -11,7 +11,8 @@ obj-$(CONFIG_PCI) += pci.o
 # Serial port support
 #
 obj-$(CONFIG_EARLY_PRINTK) += early_printk.o
-obj-$(CONFIG_SERIAL_8250) += serial.o
+loongson-serial-$(CONFIG_SERIAL_8250) := serial.o
+obj-y += $(loongson-serial-m) $(loongson-serial-y)
 obj-$(CONFIG_LOONGSON_UART_BASE) += uart_base.o
 obj-$(CONFIG_LOONGSON_MC146818) += rtc.o