drm/vc4: Add HDMI audio support

The HDMI encoder IP embeds all needed blocks to output audio, with a
custom DAI called MAI moving audio between the two parts of the HDMI
core.  This driver now exposes a sound card to let users stream audio
to their display.

Using the hdmi-codec driver has been considered here, but MAI meant
having to significantly rework hdmi-codec, and it would have left
little shared code with the I2S mode anyway.

The encoder requires that the audio be SPDIF-formatted frames only,
which alsalib will format-convert for us.

This patch is the combined work of Eric Anholt (initial register setup
with a separate dmaengine driver and using simple-audio-card) and
Boris Brezillon (moving it all into HDMI, massive debug to get it
actually working), and which Eric has the permission to release.

v2: Drop "-audio" from sound card name, since that's already implied
    (suggestion by Boris)

Signed-off-by: Eric Anholt <[email protected]>
Acked-by: Boris Brezillon <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

drivers/gpu/drm/vc4/Kconfig

@@ -2,11 +2,15 @@ config DRM_VC4
 	tristate "Broadcom VC4 Graphics"
 	depends on ARCH_BCM2835 || COMPILE_TEST
 	depends on DRM
+	depends on SND && SND_SOC
 	depends on COMMON_CLK
 	select DRM_KMS_HELPER
 	select DRM_KMS_CMA_HELPER
 	select DRM_GEM_CMA_HELPER
 	select DRM_PANEL
+	select SND_PCM
+	select SND_PCM_ELD
+	select SND_SOC_GENERIC_DMAENGINE_PCM
 	select DRM_MIPI_DSI
 	help
 	  Choose this option if you have a system that has a Broadcom