AMD Steppe Eagle: Disable "No Snoop Enable" to stop HDMI audio stutter

Ubuntu's HDMI audio has noise and echo. Disable NoSnoopEnable can
resolve this issue.  The posted amd_late_init.c northbridge code
is missing a test for Steppe Eagle northbridges.  See coreboot Gerrit
change 3934, commit ID 4ca721399c (AMD Olive Hill: Disable
NoSnoopEnable to fix HDMI audio corruptions with Ubuntu).

Change-Id: I89894d0ce4ad72ea16d61b445edb9e67920bca24
Signed-off-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Reviewed-on: http://review.coreboot.org/6822
Tested-by: build bot (Jenkins)
Reviewed-by: WANG Siyuan <wangsiyuanbuaa@gmail.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>