bcm2835-i2s: Enable MMAP support via a DT property

Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit 7ee829f ("bcm2708-i2s:
Enable MMAP support via a DT property and overlay")

The i2s driver used to claim to support MMAP, but that feature was disabled
when some problems were found. Add the ability to enable this feature
through Device Tree, using the i2s-mmap overlay.

See: #1004

Signed-off-by: Matthias Reichl <hias@horus.com>

sound/soc/bcm/bcm2835-i2s.c

@@ -802,7 +802,7 @@ static const struct snd_soc_component_driver bcm2835_i2s_component = {
 	.name		= "bcm2835-i2s-comp",
 };
 
-static const struct snd_pcm_hardware bcm2835_pcm_hardware = {
+static struct snd_pcm_hardware bcm2835_pcm_hardware = {
 	.info			= SNDRV_PCM_INFO_INTERLEAVED |
 				  SNDRV_PCM_INFO_JOINT_DUPLEX,
 	.formats		= SNDRV_PCM_FMTBIT_S16_LE |
@@ -829,6 +829,11 @@ static int bcm2835_i2s_probe(struct platform_device *pdev)
 	struct regmap *regmap[2];
 	struct resource *mem[2];
 
+	if (of_property_read_bool(pdev->dev.of_node, "brcm,enable-mmap"))
+		bcm2835_pcm_hardware.info |=
+			SNDRV_PCM_INFO_MMAP |
+			SNDRV_PCM_INFO_MMAP_VALID;
+
 	/* Request both ioareas */
 	for (i = 0; i <= 1; i++) {
 		void __iomem *base;