timers: Add struct member description for timer_base

timer_base struct lacks description of struct members. Important struct
member information is sprinkled in comments or in code all over the place.

Collect information and write struct description to keep track of most
important information in a single place.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240123164702.55612-5-anna-maria@linutronix.de