clk: qcom: common: fix legacy board-clock registration

Make sure to search only the child nodes of "/clocks", rather than the
whole device-tree depth-first starting at "/clocks" when determining
whether to register a fixed clock in the legacy board-clock registration
helper.

Fixes: ee15faff ("clk: qcom: common: Add API to register board clocks backwards compatibly")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>