MIPS: Loongson: Do not register 8250 platform device from module.

If CONFIG_SERIAL_8250 is set to m, the Loongson seria.ko module might get
unloaded while the serial driver modules are still loaded resulting in
stale references to the destroyed platform_device instance.

Anyway, platform devices should always be registered indicated what
devices are present, _not_ what drivers have been configured.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Patchwork: https://patchwork.linux-mips.org/patch/10538/