Fix AMD Fam14 cbmen allocation

Fix AMD Fam14 cbmen allocation

The Fam14 northbridge.c had hardcoded the cbmem size. It should use
in cbmem.h instead.

Change-Id: I910329fc98a4cf04dc81ef66f3aa05a1916f5b1d
Signed-off-by: Marc Jones <marc.jones@se-eng.com>
Reviewed-on: http://review.coreboot.org/790
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins)