ARM: shmobile: bockw: add CN9 SCIF/RCAN selection dipswitch explanation

Debug serial (= SCIF0) is connected to CN9 upper side,
and it is shared by RCAN.
This patch adds SCIF/RCAN dipswitch explanation on
comment area for developers.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>