arm64: dts: allwinner: a64: Add default dr_mode

The USB OTG binding we have mandates to have a dr_mode property, yet not
all boards are setting it.

Since the generic otg binding states that the default mode should be the
OTG mode, let's use that one in our DTSI.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>