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

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

Patch Review

Clean refactoring. The new `of_reserved_mem_device_init_with_mem()` extracts the core logic from `of_reserved_mem_device_init_by_idx()`.

**Minor: NULL check change in behavior**

The original code in `of_reserved_mem_device_init_by_idx()` checked:
```c
	if (!rmem || !rmem->ops || !rmem->ops->device_init)
		return -EINVAL;
```

Now `of_reserved_mem_device_init_by_idx()` calls the new helper which does these checks, but `of_reserved_mem_lookup()` can return NULL, and the new helper also checks `!rmem`, so the behavior is preserved. The stub for `!CONFIG_OF_RESERVED_MEM` returns `-EOPNOTSUPP` instead of the previous `-ENOSYS` (from `of_reserved_mem_device_init_by_idx`'s stub) — this is fine and arguably more correct.

No issues found. Well done.

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