init_moto_msm8960: Add support for AT&T Atrix HD variant

AT&T Atrix HD bootloader does not seem to set ro.boot.modelno property
at all.

(The no longer used gprs>edge override property is silently removed
in this commit as well, just by the way.)

Change-Id: I7d40bb8d91d2e22986a94d55895daaf71151004a