clk: at91: fix clk-generated parenting

clk_generated_startup is called after clk_hw_register. So the first call to
get_parent will not have the correct value (i.e. 0) and because this is
cached, it may never be updated.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Fixes: df70aeef ("clk: at91: add generated clock driver")
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>