docs: dt: Make 'Devicetree' wording more consistent

There's a variety of ways 'Devicetree' has been written. This is most
evident in the documentation build contents where we have 'Device Tree',
'DeviceTree', etc. The DT spec has somewhat standardized on
'Devicetree', so let's use that.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-5-robh@kernel.org