soc/intel/braswell: Add SoC stepping identify helper

Adapted from Chromium commit 9756af8.

Add SOC helper to identify BSW SoC stepping.  Will be used to
override USB2 phy setting based on stepping in subsequent commit.

Original-Change-Id: Ic736dd945f01cf9f24af4ce3bd3f2757abfdeb2e
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Tested-by: Keith Tzeng <keith.tzeng@quantatw.com>

Change-Id: Ib2371f85ea84df4b417e25ec8840d317cd918d5f
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/21371
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
2 files changed