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

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.com
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>