MIPS: dts: loongson: Add ISA node

ISA node is required by Loongson64 platforms to initialize
PIO support.

Kernel will hang at boot without ISA node.

Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>