clk: gate: Add some kunit test suites

Test various parts of the clk gate implementation with the kunit testing
framework.

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Daniel Latypov <dlatypov@google.com>
Cc: <kunit-dev@googlegroups.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20220120175902.2165958-1-sboyd@kernel.org