rust: time: Seal the HrTimerMode trait

rust: time: Seal the HrTimerMode trait

Prevent downstream crates or drivers from implementing `HrTimerMode`
for arbitrary types, which could otherwise leads to unsupported
behavior.

Introduce a `private::Sealed` trait and implement it for all types
that implement `HrTimerMode`.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250617232806.3950141-1-fujita.tomonori@gmail.com
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>