Add documentation of the Device Tree bindings for the Image Processing
Unit (IPU) found in most Ingenic SoCs.

v2: Add missing 'const' in items list
v3: No change
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-3-paul@crapouillou.net