riscv: dts: thead: set dma-noncoherent to soc bus

riscv select ARCH_DMA_DEFAULT_COHERENT by default, and th1520 isn't
dma coherent, so set dma-noncoherent to reflect this fact.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Tested-by: Drew Fustini <dfustini@baylibre.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/riscv/boot/dts/thead/th1520.dtsi

@@ -139,6 +139,7 @@ soc {
 		interrupt-parent = <&plic>;
 		#address-cells = <2>;
 		#size-cells = <2>;
+		dma-noncoherent;
 		ranges;
 
 		plic: interrupt-controller@ffd8000000 {