kern: efirt: correct configuration table entry size

Each entry actually stores a native pointer, not a uint64_t quantity. While
we're here, go ahead and export the pointer as-is rather than converting it
to KVA. This may be more useful as consumers can map /dev/mem and observe
the entry.

For reference, see: sys/contrib/edk2/Include/Uefi/UefiSpec.h

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D27669