arm64: dts: mediatek: Makefile: Generate symbols for DTBO support

Add DTC_FLAGS '-@' for mt7986a-bananapi-bpi-r3 and -mini to
instruct the devicetree compiler to enable generation of symbols.

This allows proper support for Device Tree Overlay(s) for those
boards; future boards that need DTBO support are expected to add
their own DTC_FLAGS_{dtb-name}.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240620101830.1097548-1-angelogioacchino.delregno@collabora.comSigned-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

arch/arm64/boot/dts/mediatek/Makefile

@@ -84,3 +84,7 @@ dtb-$(CONFIG_ARCH_MEDIATEK) += mt8390-genio-700-evk.dtb
 dtb-$(CONFIG_ARCH_MEDIATEK) += mt8395-kontron-3-5-sbc-i1200.dtb
 dtb-$(CONFIG_ARCH_MEDIATEK) += mt8395-radxa-nio-12l.dtb
 dtb-$(CONFIG_ARCH_MEDIATEK) += mt8516-pumpkin.dtb
+
+# Device tree overlays support
+DTC_FLAGS_mt7986a-bananapi-bpi-r3 := -@
+DTC_FLAGS_mt7986a-bananapi-bpi-r3-mini := -@