Platform-only build of gftv100 doesn't use BCM_NEXUS.

We need a different way to detect which gpio-mailbox pinout to use.  Let's
do what we ended up doing with the other platforms, and just hardcode it by
model number.  b/33070819

Change-Id: If1dc712149081355a1cd7add9342e26d94a3985e
1 file changed