MIPS: Loongson: Remove the serial port output of compressed kernel support

MIPS: Loongson: Remove the serial port output of compressed kernel support

The compressed kernel support on loongson family machines is stable now,
so, remove the debug information via using SYS_SUPPORTS_ZBOOT instead of
SYS_SUPPORTS_ZBOOT_UART16550. This may reduce the image size and speedup
the booting.

Signed-off-by: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: yanh@lemote.com
Cc: huhb@lemote.com
Cc: zhangfx@lemote.com
Patchwork: http://patchwork.linux-mips.org/patch/824/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>