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/dma-coherence.h

@@ -15,7 +15,9 @@ enum coherent_io_user_state {
 	IO_COHERENCE_DISABLED,
 };
 
-#ifdef CONFIG_DMA_MAYBE_COHERENT
+#if defined(CONFIG_DMA_PERDEV_COHERENT)
+/* Don't provide (hw_)coherentio to avoid misuse */
+#elif defined(CONFIG_DMA_MAYBE_COHERENT)
 extern enum coherent_io_user_state coherentio;
 extern int hw_coherentio;
 #else