docs: arm: index.rst: add google/chromebook-boot-flow

This document was added without placing it at arm book.

Fixes: 59228d3b ("dt-bindings: Document how Chromebooks with depthcharge boot")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/0ae8251f97c642cfd618f2e32eb1e66339e5dfde.1656759989.git.mchehab@kernel.org

Documentation/arm/index.rst

@@ -31,6 +31,8 @@ SoC-specific documents
 .. toctree::
    :maxdepth: 1
 
+   google/chromebook-boot-flow
+
    ixp4xx
 
    marvell