powerpc/64: Mark prep_irq_for_idle() __cpuidle

Code in the idle path is not allowed to be instrumented because RCU is
disabled, see commit 0e985e9d ("cpuidle: Add comments about
noinstr/__cpuidle usage").

Mark prep_irq_for_idle() __cpuidle, which is equivalent to noinstr, to
enforce that.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230406144535.3786008-1-mpe@ellerman.id.au