ARM: shmobile: r8a7790: add main clock

Almost all clock needs main clock which is basis clock on r8a7790.
This patch adds it, and, set its parent/ratio via MD pin.
It is based on v0.05 datasheet

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