ARM: tegra: nexus7: Enable USB OTG mode

Nexus 7 has OTG-cable microUSB port, enable OTG mode. USB peripheral
devices now can be connected to Nexus 7 using OTG adapter, switching
USB port into host mode.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>