MIPS: Support per-device DMA coherence

On some MIPS systems, a subset of devices may have DMA coherent with CPU
caches. For example in systems including a MIPS I/O Coherence Unit
(IOCU), some devices may be connected to that IOCU whilst others are
not.

Prior to this patch, we have a plat_device_is_coherent() function but no
implementation which does anything besides return a global true or
false, optionally chosen at runtime. For devices such as those described
above this is insufficient.

Fix this by tracking DMA coherence on a per-device basis with a
dma_coherent field in struct dev_archdata. Setting this from
arch_setup_dma_ops() takes care of devices which set the dma-coherent
property via device tree, and any PCI devices beneath a bridge described
in DT, automatically.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14349/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/mach-generic/dma-coherence.h

@@ -49,6 +49,9 @@ static inline int plat_dma_supported(struct device *dev, u64 mask)
 
 static inline int plat_device_is_coherent(struct device *dev)
 {
+#ifdef CONFIG_DMA_PERDEV_COHERENT
+	return dev->archdata.dma_coherent;
+#else
 	switch (coherentio) {
 	default:
 	case IO_COHERENCE_DEFAULT:
@@ -58,6 +61,7 @@ static inline int plat_device_is_coherent(struct device *dev)
 	case IO_COHERENCE_DISABLED:
 		return 0;
 	}
+#endif
 }
 
 #ifndef plat_post_dma_flush