clk: remove unnecessary !core->parents conditional

This if-block has been here since the introduction of the common
clock framework.  Now no clock drivers are statically initialized.
core->parent is always NULL at this point.  Drop the redundant
check and the confusing comment.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>