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

Since 635c21068cf ("usb: dwc2: gadget: Fix WARN_ON messages
during FIFO init") the dwc2 driver is able to handle OTG and gadget
mode for bcm2835. So enable this feature for the Raspberry Pi Zero.

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