dts: meson-gxbb: Drop redundant devicetree files

Since meson-gxbb based boards switched to using upstream DT, so drop
redundant files from arch/arm/dts directory. Only *-u-boot.dtsi files
kept in arch/arm/dts directory for these boards.

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