.gitignore: move *.dtb and *.dtb.S patterns to the top-level .gitignore

Most of DT files are compiled under arch/*/boot/dts/, but we have some
other directories, like drivers/of/unittest-data/.  We often miss to
add gitignore patterns per directory.  Since there are no source files
that end with .dtb or .dtb.S, we can ignore the patterns globally.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>