AMD Steppe Eagle: Add northbridge files for new SoC family

Add the northbridge file for AMD's new Mullins and Steppe Eagle
processor family.  Since the processor family name is not the
same across AMD's sales and marketing channels, I have elected
to use part of the processor ID as the family name.  The intent
is to reduce confusion since the processor ID is the same for
both families.  This northbridge support has only been validated
on the AMD Embedded variants ("Steppe Eagle").

The AGESA wrappers in coreboot have a function that is intended to
mirror the UMA memory allocation performed during memory initialization
by AGESA.  Update the Steppe Eagle memory allocation to mimic the
memory reservation done inside the AGESA BLOB.

Change the default CBMEM address, the default video BIOS device ID,
and a couple of other defaults to match changes in coreboot community
code.

The northbridge chip.h specifies how many processor sockets, how
many channels, and how many DIMM slots are supported by the
northbridge.  Steppe Eagle does not permit multisocket systems
and has only one memory controller channel.

Change-Id: I20d8b78e3b153cda2dd05100fbb75e2ebadd9e08
Signed-off-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Reviewed-on: http://review.coreboot.org/6678
Tested-by: build bot (Jenkins)
Reviewed-by: WANG Siyuan <wangsiyuanbuaa@gmail.com>
Reviewed-by: Zheng Bao <zheng.bao@amd.com>
17 files changed