LoongArch: Add processing ISA Node in DeviceTree

Similar to commit 6d0068ad ("MIPS: Loongson64: Process ISA
Node in DeviceTree"), we process ISA node in DeviceTree for FDT-based
systems.

Previously, we are hardcoding reserved ISA I/O Space in, now we are
processing it I/O via DeviceTree directly. The ranges property of ISA
node is used to determine the size and address of reserved I/O space.

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