.gitattributes: use 'dts' diff driver for dts files

Git is gaining support to display the closest node to the diff in the
hunk header via the 'dts' diff driver.  Use that driver for all dts and
dtsi files so we can gain some more context on where the diff is.
Taking a recent commit in the kernel dts files you can see the
difference.

With this patch and an updated git

: diff --git a/arch/arm64/boot/dts/nvidia/tegra194-p2888.dtsi b/arch/arm64/boot/dts/nvidia/tegra194-p2888.dtsi
:  index 62e07e1197cc..4c38426a6969 100644
:  --- a/arch/arm64/boot/dts/nvidia/tegra194-p2888.dtsi
:  +++ b/arch/arm64/boot/dts/nvidia/tegra194-p2888.dtsi
:  @@ -289,5 +289,29 @@ vdd_hdmi: regulator@1 {
:                          gpio = <&gpio TEGRA194_MAIN_GPIO(A, 3) GPIO_ACTIVE_HIGH>;
:                          enable-active-high;
:                  };
:  +
:  +               vdd_3v3_pcie: regulator@2 {
:  +                       compatible = "regulator-fixed";

vs. without this patch

:  diff --git a/arch/arm64/boot/dts/nvidia/t...