ravb: update "undocumented" annotations

The "undocumented" annotations in the EtherAVB driver were done against
the R-Car gen2 manuals; most of these registers/bits were then described
in the R-Car gen3 manuals -- reflect  this fact in the annotations (note
that ECSIPR.LCHNGIP was documented in the recent R-Car gen2 manual)...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omprussia.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>