genirq/timings: Add kernel-doc for a function parameter

Add the description for @now to eliminate a kernel-doc warning.

timings.c:537: warning: Function parameter or struct member 'now' not described in 'irq_timings_next_event'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250111062954.910657-1-rdunlap@infradead.org