userdiff: add a builtin pattern for dts files

userdiff: add a builtin pattern for dts files

The Linux kernel receives many patches to the devicetree files each
release. The hunk header for those patches typically show nothing,
making it difficult to figure out what node is being modified without
applying the patch or opening the file and seeking to the context. Let's
add a builtin 'dts' pattern to git so that users can get better diff
output on dts files when they use the diff=dts driver.

The regex has been constructed based on the spec at devicetree.org[1]
and with some help from Johannes Sixt.

[1] https://github.com/devicetree-org/devicetree-specification/releases/latest

Cc: Rob Herring <redacted>
Cc: Frank Rowand <redacted>
Signed-off-by: Stephen Boyd <redacted>
Signed-off-by: Junio C Hamano <redacted>