Only return a mapped address from efi_phys_to_kva

On some hardware the EFI system table is not in memory mapped in the
DMAP section. Rather than panic the kernel check if it is mapped and
return a failure if not from efi_phys_to_kva.

Reported by: kevans
Differential Revision: https://reviews.freebsd.org/D34858

(cherry picked from commit f3ef799f564515b58a24a328739a4c749a02b110)