arm64: dts: ls1028a: default to OTG mode for USB

At the moment, the dtsi will force the dr_mode to host. This is
problematic because it will always turn on the Vbus voltage regardless
if the port is host or device. This might lead to a "shortcut" between
the two USB endpoints because both might have their Vbus supplies
enabled. Therefore, the default should be "otg" for any ports which
aren't host only (from a SoC point of view) and have a user of the dtsi
file overwrite that explicitly.

Move the 'dr_mode = "host";' into the board dts. Now that the dtsi
doesn't set the dr_mode anymore, we can also drop the 'dr_mode = "otg";'
in the board dts because that is the default value if dr_mode is not
set.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>