sched/deadline: Clarify nanoseconds in uapi

Specify the time values of the deadline parameters of deadline,
runtime, and period as being in nanoseconds explicitly as they always
have been.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20240813144348.1180344-3-christian.loehle@arm.com