Include the EFI Runtime Code in the DMAP

Some UEFI implementations place the system table in a runtime code
memory region. Include it in the DMAP so we can read it later.

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D34861

(cherry picked from commit 57e47ae514b6ab605298b4595e0219048f59ab7a)