MIPS: Loongson{2ef, 32, 64} convert to generic fw cmdline

All of Loongson firmwares are passing boot cmdline/env
in the manner of YAMON/PMON. Thus we can remove duplicated
cmdline initialize code and convert to generic fw method.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: chenhe@lemote.com