northbridge/intel/i945: Add define for register `BSM` and use it

Add a define for the register Base of Stolen Memory (BSM) and use it.

Change-Id: I5b1df4e088d88344fac8cd8d218e76b08a885f58
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/5884
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>