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.