timers/migration: Use a single struct for hierarchy walk data

Two different structs are defined for propagating data from one to another
level when walking the hierarchy. Several struct members exist in both
structs which makes generalization harder.

Merge those two structs into a single one and use it directly in
walk_groups() and the corresponding function pointers instead of
introducing pointer casting all over the place.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20240716-tmigr-fixes-v4-4-757baa7803fe@linutronix.de