device-tree: nexus7: Set phy mode to otg instead of host

In order to have the usb phy work in gadget mode, override
the default host mode with otg mode.

This allows gadget mode to work w/o any hacks to the dtsi file.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Andy Gross <agross@codeaurora.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>