E820: Add interface for accessing e820 table

Add the following two functions:

  - e820_get_num_entries() - query the size of the e820 table
  - e820_get_entry() - grab an entry matching a given set of criteria

This interface is currently necessary for creating type 19
(memory array mapped address) structures in smbios.

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