clocksource: Cleanup struct clocksource and documentation

Reformat the struct definition, add missing member documentation.
No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/20200207124402.825471920@linutronix.de