INTERNAL: Fix USB OTG for Mistral platform

The OTG as peripheral requires extcon to be set,
this patch enables the same in the mistral dts.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Khasim Syed Mohammed <khasim.mohammed@linaro.org>
Tested-by: Praveen Koya <c_pkoya@qti.qualcomm.com>
Change-Id: Ibefab5553384740a55ee3fb1f1c33936eae1a077