Claude review: of_reserved_mem: add a helper for rmem device_init op

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Adds `of_reserved_mem_device_init_with_mem()` that calls `rmem->ops->device_init()` directly, and refactors `of_reserved_mem_device_init_by_idx()` to use it.

**Issues:**

1. **Memory leak introduced in `of_reserved_mem_device_init_by_idx()`.** The original code checks `if (!rmem || !rmem->ops || !rmem->ops->device_init) return -EINVAL;` **before** the `kmalloc_obj()`. The refactored code moves the validation into `of_reserved_mem_device_init_with_mem()`, but now `rd` is allocated first:
```c
-	if (!rmem || !rmem->ops || !rmem->ops->device_init)
-		return -EINVAL;
-
 	rd = kmalloc_obj(struct rmem_assigned_device);
 	if (!rd)
 		return -ENOMEM;

-	ret = rmem->ops->device_init(rmem, dev);
+	ret = of_reserved_mem_device_init_with_mem(dev, rmem);
```
If `rmem` is NULL or has no ops, the new helper returns `-EINVAL`, but `rd` was already allocated and is only freed in the `else` branch of `if (ret == 0)`. Looking at the original code after the hunk: `if (ret == 0) { ... } else { kfree(rd); }` -- so `rd` does get freed on the error path. This is actually fine. However, there's a subtler issue: the original code never reached `kmalloc_obj()` when `rmem` was NULL, but now it does, wasting memory only to immediately free it. Not a bug per se, but a minor inefficiency change.

2. **Good:** The `#ifndef CONFIG_OF_RESERVED_MEM` stub returns `-EOPNOTSUPP` which is appropriate.

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