smbios: support getting address from EFI

On some systems (e.g. Lenovo ThinkPad X240, Apple MacBookPro12,1)
the SMBIOS entry point is not found in the <0xFFFFF space.

Follow the SMBIOS spec and use the EFI Configuration Table for
locating the entry point on EFI systems.

Reviewed by: rpokala, dab
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D29276