SMBIOS: Update header file definitions

Add definitions for smbios entry point (anchor), and for type 2
(base board) structure which is required by some versions of OS X.

Remove definition for type 20 (memory device mapped address)
structure, which is no longer required as of smbios spec v2.5.

Update all other structure definitions to bring them into
compliance with smbios spec v2.8.

This patch contains no functional changes.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>