sh: clkfwk: setup clock parent from current register value

Some clocks can select its parent clock by CPG register.
But it might have been modified by boot-loader or something.
This patch removed fixed initial parent clock,
and setup it from their current register settings.
It works on div6 reparent clocks for now.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>