Automatically fetch bus information for mptable from

Automatically fetch bus information for mptable from
the device tree, instead of using hardcoded values.

If this changes behaviour, this is either
- a bug in mptable_write_buses(), or
- a bug in the old mptable or device config, that is
  they were inconsistent.

Signed-off-by: Patrick Georgi <patrick.georgi@coresystems.de>
Acked-by: Peter Stuge <peter@stuge.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5835 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1