efirt: Don't enter EFI context early, convert addrs to KVA instead

efi_enter here was needed because efi_runtime dereference causes a fault
outside of EFI context, due to runtime table living in runtime service
space. This may cause problems early in boot, though, so instead access it
by converting paddr to KVA for access.

While here, remove the other direct PHYS_TO_DMAP calls and the explicit DMAP
requirement from efidev.

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D16591