MIPS: Loongson1B: Improve early printk

- Determine serial port for early printk according to kernel command line.
  - Move to 8250/16550 serial early printk driver.

Signed-off-by: Kelvin Cheung <keguang.zhang@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8023/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>