sh: clkfwk: clock-sh73a0: all div6_clks use SH_CLK_DIV6_EXT()

Current div6 clocks can specify their current parent clocks
from its register value if it is registered
by sh_clk_div6_reparent_register().
This patch modifies all div6 clocks into SH_CLK_DIV6_EXT().

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