media: docs: move fourcc file to the uAPI document

This file gives a hint about how fourcc should be named.

It is on a very weird place, as such kind of thing belongs to the
uAPI guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/common.rst

@@ -57,6 +57,7 @@ applicable to all devices.
     ext-ctrls-fm-tx
     ext-ctrls-fm-rx
     ext-ctrls-detect
+    fourcc
     format
     planar-apis
     selection-api

Documentation/userspace-api/media/v4l/fourcc.rst

@@ -0,0 +1,32 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Guidelines for Video4Linux pixel format 4CCs
+============================================
+
+Guidelines for Video4Linux 4CC codes defined using v4l2_fourcc() are
+specified in this document. First of the characters defines the nature of
+the pixel format, compression and colour space. The interpretation of the
+other three characters depends on the first one.
+
+Existing 4CCs may not obey these guidelines.
+
+Raw bayer
+---------
+
+The following first characters are used by raw bayer formats:
+
+- B: raw bayer, uncompressed
+- b: raw bayer, DPCM compressed
+- a: A-law compressed
+- u: u-law compressed
+
+2nd character: pixel order
+
+- B: BGGR
+- G: GBRG
+- g: GRBG
+- R: RGGB
+
+3rd character: uncompressed bits-per-pixel 0--9, A--
+
+4th character: compressed bits-per-pixel 0--9, A--