tftpboot: set phram erase block size to match hardware

this is needed for tftp boot with a jffs2 filesystem

devices/gl-mt300a/default.nix

@@ -45,6 +45,7 @@
         flash = {
           address = "0xbc050000";
           size ="0xf80000";
+          eraseBlockSize = "65536";
         };
         rootDevice = "1f05";
 

devices/gl-mt300n-v2/default.nix

@@ -42,6 +42,7 @@
         flash = {
           address = "0xbc050000";
           size = "0xfb0000";
+          eraseBlockSize = "65536";
         };
         rootDevice = "1f05";