aarch64: make tftpboot work

- patch dtb to add reserved-memory stanza for the phram device to use
  (aarch64 does not accept memmap= command line option)

- patch phram driver to use memremap() instead of ioremap() as
  ioremap can't be used for system ram on arm devices

pkgs/kernel/default.nix

@@ -54,6 +54,7 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./cmdline-cookie.patch
+    ./phram-allow-cached-mappings.patch
   ];
 
   # this is here to work around what I think is a bug in nixpkgs