dts: msm8996: replace with upstream DTS

Drop the U-Boot specific dragonboard820c.dts file in favour of the
upstream apq8096-db820c.dts and an additional -u-boot.dtsi with the
U-Boot specific additions.

Taken from kernel tag v6.7

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>