timekeeping: Add a lockdep override in tick_freeze()

tick_freeze() acquires a raw spinlock (tick_freeze_lock). Later in the
callchain (timekeeping_suspend() -> mc146818_avoid_UIP()) the RTC driver
acquires a spinlock which becomes a sleeping lock on PREEMPT_RT.  Lockdep
complains about this lock nesting.

Add a lockdep override for this special case and a comment explaining
why it is okay.

Reported-by: Borislav Petkov <bp@alien8.de>
Reported-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20250404133429.pnAzf-eF@linutronix.de
Closes: https://lore.kernel.org/all/20250330113202.GAZ-krsjAnurOlTcp-@fat_crate.local/
Closes: https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@mail.gmail.com/