ARM: shmobile: r8a73a4: add main clock

Almost all clock needs main clock which is basis clock on r8a73a4.
This patch adds it, and, set parent clock via CKSCR register.

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