efi_loader: Respect DT reserved regions

With legacy boot (booti, bootz), people can declare memory regions as
reserved using device tree memory reservations. This feature is some
times used to indicate memory regions that should not be touched.

Since in a UEFI world, the DT memory reservations do not get honored,
let's copy them into the UEFI memory map so everyone has a coherent
view of the world and we give people the chance to add reservations
on demand.

Signed-off-by: Alexander Graf <agraf@suse.de>