clk: renesas: rcar-gen3: Add documentation for SD clocks

Document the known use cases of the different clock settings. This is
useful as different SoC and ES versions use different settings to do
the same thing as there is more than one combination to achieve the
same SDn clock speed.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>