ARM: dts: bcm283x: Add missing USB clock

According to the DWC2 binding an appropriate clock is required.
This clock isn't handled by bcm2835 clock driver, so add a fixed
clock to the bcm283x DT.

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