LoongArch: Cleanup headers to avoid circular dependency

When enable GENERIC_IOREMAP, there will be circular dependency to cause
build errors. The root cause is that pgtable.h shouldn't include io.h
but pgtable.h need some macros defined in io.h. So cleanup those macros
and remove the unnecessary inclusions, as other architectures do.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>