ARM: shmobile: sh73a0: use fixed ratio clock

Current clock-sh73a0 is using own implement
for each divX clocks.
This patch switches to use fixed ratio clock,
and was tesed on kzm9g board.

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