clk: Use lockdep asserts to find missing hold of prepare_lock

Add lockdep asserts for holding the prepare_lock to all functions
marking this as a requirement in description. Add this to private and
exported functions so all locking misuse could be detected during
debugging.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>