MIPS: Loongson: Cleanup of the environment variables

Changes:

	o Move bus_clock into prom_init_env()
	o Initialize the cpu_clock_freq to the default values for the
	correspoding processor revisions if no such environment variable
	passed by BIOS/Bootloader.

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/826/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>