arm64: dts: mt8173-elm: fix supported values for regulator-allowed-modes of da9211

According to the datasheet the allowed modes for the da9211
regulator are sync and auto mode. This should be changed in the
devicetree. This also fix an error message
'BUCKA: invalid regulator-allowed-modes element 0'
since value 0 is invalid.

Fixes: 689b937b

 ("arm64: dts: mediatek: add mt8173 elm and hana board")
Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200903142819.24487-1-dafna.hirschfeld@collabora.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>