AMD Steppe Eagle: Update reference to BLOBs repo (3rdparty)

The BLOBs repo has been updated with AMD PI header files, peripheral
BLOBs for the new Avalon southbridge, the AGESA binary PI BLOB for
Steppe Eagle, the Steppe Eagle video BIOS, and platform security
processor firmware.

Change-Id: I8bb58a5cc572d2d75de33b14843d7d1893fff532
Signed-off-by: Bruce Griffith <Bruce.Griffith@se-eng.com>
Reviewed-on: http://review.coreboot.org/6770
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>