arm64: allwinner: dts: h6: fix Pine H64 MMC bus width

Currently the enabled MMC controllers on Pine H64 do not have bus-width
set, which make them fall back to 1-bit mode and become quite slow.

Fix this by add the corresponding bus-width properties.

Fixes: ecbd6118 ("arm64: allwinner: h6: enable MMC0/2 on Pine H64")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>