clk: fix clock tree dump to properly dump out every registered clock

Some clocks are not associated to a DM node, so just parsing the DM is not
enough. This is especially true for root clocks, which typically don't have
any parents. Instead, fetch every registered UCLASS_CLK instance, and dump
these out.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>