ARM: dts: bcm2835-rpi-zero-w: Enable OTG mode

Since commit 9273083a ("usb: dwc2: Fix TxFIFOn sizes and total TxFIFO
size issues") we could use the OTG mode again.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>