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>

diff --cc arch/mips/loongson1/Kconfig
Simple merge

diff --cc arch/mips/loongson1/common/prom.c
Simple merge