ipmi: support getting address from EFI

The original implementation only supports getting the address from legacy
BIOS (by searching for the SMBIOS_SIG pattern in a fixed address space).

Try to get the SMBIOS table from EFI through efirt (EFI Runtime Services)
firstly. Continue to search in the legacy BIOS if a NULL address is
returned from EFI.

By this way the ipmi function supports both legacy BIOS and UEFI systems.

Reviewed by: dab, vangyzen
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D30007