soc: loongson: loongson2_guts: Convert to devm_platform_ioremap_resource()

Use devm_platform_ioremap_resource() to simplify code.

Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>