LoongArch: Use correct API to map cmdline in relocate_kernel()

LoongArch: Use correct API to map cmdline in relocate_kernel()

[ Upstream commit 0124fbb4c6dba23dbdf80c829be68adbccde2722 ]

fw_arg1 is in memory space rather than I/O space, so we should use
early_memremap_ro() instead of early_ioremap() to map the cmdline.
Moreover, we should unmap it after using.

Suggested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>