dts: meson: Drop redundant GXL, GXM & AXG devicetree files

dts: meson: Drop redundant GXL, GXM & AXG devicetree files

Since meson GXL, GXM & AXG 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.

Cc: Sumit Garg <sumit.garg@linaro.org>
Acked-by: Viacheslav Bocharov <adeep@lexina.in>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # khadas-vim3_android
Link: https://lore.kernel.org/r/20240329-u-boot-of-upstream-v2-3-2512ad3eb63d@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>