efi_loader: Expose ascending efi memory map

The EFI memory map does not need to be in a strict order, but 32bit
grub2 does expect it to be ascending. If it's not, it may try to
allocate memory inside the U-Boot data memory region.

We already sort the memory map in descending order, so let's just
reverse it when we pass it to a payload.

Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>