rust: opp: Move `cfg(CONFIG_OF)` attribute to the top of doc test

Move the `#[cfg(CONFIG_OF)]` attribute to the top of the documentation test
block and hide it. This applies the condition to the entire test and improves
readability.

Placing configuration flags like `CONFIG_OF` at the top serves as a clear
indicator of the conditions under which the example is valid, effectively
acting like configuration metadata for the example itself.

Suggested-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/9d93c783cc4419f16dd8942a4359d74bc0149203.1748323971.git.viresh.kumar@linaro.org


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>