riscv: fix issue of no DMA buffer for eth

* By default, dev.coherent_dma_mask is DMA_BIT_MASK(32) for all devices.
  But when we put the kernel over 4GB physical memory, devices can not
  get DMA buffer with this default mask. Setting this mask to
  DMA_BIT_MASK(64) can fix this issue. Note that by default
  arch_setup_pdev_archdata() is defined as a weak function.

arch/riscv/mm/Makefile

@@ -3,3 +3,4 @@ obj-y += fault.o
 obj-y += extable.o
 obj-y += ioremap.o
 obj-y += cacheflush.o
+obj-y += dma.o

arch/riscv/mm/dma.c

+#include <linux/dma-mapping.h>
+#include <linux/platform_device.h>
+
+void arch_setup_pdev_archdata(struct platform_device *pdev)
+{
+	if (pdev->dev.coherent_dma_mask == DMA_MASK_NONE &&
+	    pdev->dev.dma_mask == NULL) {
+		pdev->dev.coherent_dma_mask = DMA_BIT_MASK(64);
+		pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
+	}
+}