hrtimers: Move hrtimer base related definitions into hrtimer_defs.h

hrtimer base related struct definitions are part of hrtimers.h as it is
required there. With this, also the struct documentation which is for core
code internal use, is exposed into the general api.

To prevent this, move all core internal definitions and the related
includes into hrtimer_defs.h.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240123164702.55612-2-anna-maria@linutronix.de