efi_loader: fix memory type for memory reservation block

The (yet unreleased version of the) devicetree specification clearly
states that:

  As with the /reserved-memory node, when booting via UEFI
  entries in the Memory Reservation Block must also be listed
  in the system memory map obtained via the GetMemoryMap() toi
  protect against allocations by UEFI applications. The memory
  reservation block entries should be listed with type
  EfiReservedMemoryType.

This restores the behaviour that was changed by commit 4cbb2930
("efi_loader: consider no-map property of reserved memory").

Fixes: 4cbb2930 ("efi_loader: consider no-map property of reserved memory")
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>