sh: kfr2r09: document the PLL/FLL <-> RF relationship.

Early clock initialization sets up PLL/FLL values for optimal RF
behaviour. As this relationship is presently undocumented, we document
this in the script so the rationale is apparent.

Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>