bus: ti-sysc: Add missing kerneldoc comments

A few fields in various structures is missing the corresponding
kerneldoc comments. Add them. Also, fixed the comment for sidlemodes.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>