efi_loader: efidebug, avoid illegal memory access

For EFI_PERSISTENT_MEMORY_TYPE the 'efidebug memmap' command produces an
illegal memory access.

* Add the missing descriptive string for EFI_PERSISTENT_MEMORY_TYPE.
* Replace the check for EFI_MAX_MEMORY_TYPE by the ARRAY_SIZE() macro.

Reported-by: Coverity (CID 300336)
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>