arm64: dts: mediatek: mt7988a-bpi-r4: Add fixed regulators for 1v8 and 3v3

Add regulator nodes used for mmc to Bananapi R4 board.
This board has 1 MMC controller used for SDMMC and eMMC where only one can
be used at one time, selected by hardware switches. SD uses 3v3 for both
supplies and emmc uses both regulators.
So defining both regulators in board dts and referencing them in the dt
overlay.

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/20241217091238.16032-13-linux@fw-web.de


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