Add support to run SMM handler in TSEG instead of ASEG

Add support to run SMM handler in TSEG instead of ASEG

Traditionally coreboot's SMM handler runs in ASEG (0xa0000),
"behind" the graphics memory. This approach has two issues:
- It limits the possible size of the SMM handler (and the
  number of CPUs supported in a system)
- It's not considered a supported path anymore in newer CPUs.

Change-Id: I9f2877e46873ab2ea8f1157ead4bc644a50be19e
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Acked-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/842
Reviewed-by: Peter Stuge <peter@stuge.se>
Tested-by: build bot (Jenkins)