Documentation: ARM: fix reference to DT format documentation

The booting-without-of.rst file is no longer there. Link to
devicetree.org instead.

Fixes: 44184828

 ("dt: Remove booting-without-of.rst")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Cc: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/7f07e544d9fc584242d496c2f54f9303d8de0724.1611558630.git.baruch@tkos.co.il
Signed-off-by: Jonathan Corbet <corbet@lwn.net>