powerpc/85xx: set USB2 default mode to 'device' for (super)hydra boards

The Hydra and Superhydra (P3041DS, P5020DS, and P5040DS) boards have a
second USB port that can be configured in either host, peripheral (aka
device), or OTG (on-the-go) mode.  When configured in host mode, if
the port is connected to another USB host, damage to the board can
occur.

To avoid this, we change the default setting to peripheral mode.  Ideally,
we'd set it to OTG mode, but currently there is no OTG support for
these boards.

Setting the hwconfig variable will also update the device tree, and so
Linux will configure the port for peripheral mode as well.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>