ARM: dts: bcm283x: Add dtsi for OTG mode

The Raspberry Pi Zero also supports OTG mode. So provide a dtsi file
to configure the USB interface accordingly. The fifo sizes are optimized
for device endpoint 6 and 7 with the maximum of 768.

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