clk: Allow kunit tests to run without OF_OVERLAY enabled

Some configurations want to enable CONFIG_KUNIT without enabling
CONFIG_OF_OVERLAY. The kunit overlay code already skips if
CONFIG_OF_OVERLAY isn't enabled, so these selects here aren't really
doing anything besides making it easier to run the tests without them
skipping. Remove the select and move the config setting to the
drivers/clk/.kunitconfig file so that the clk tests can be run with or
without CONFIG_OF_OVERLAY set to test either behavior.

Fixes: 5776526b ("clk: Add KUnit tests for clk fixed rate basic type")
Fixes: 274aff87 ("clk: Add KUnit tests for clks registered with struct clk_parent_data")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20241016212738.897691-1-sboyd@kernel.org