arm64: dts: mediatek: mt7988a-bpi-r4: Add dt overlays for sd + emmc

Bananapi R4 can use the mmc controller either with sd card or the onboard
emmc. The used device is selected through hardware-switches and probed in
bootloader.
Add devicetree overlays for both mmc configurations.

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20241217091238.16032-14-linux@fw-web.de


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