ARM: shmobile: r8a7779: add each clocks ratio on comment area

Adding comment describing the r8a7779 clock frequencies depending on
MD pin settings.

Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>