clk: Mention that .recalc_rate can return 0 on error

clk: Mention that .recalc_rate can return 0 on error

Multiple platforms (amlogic, imx8) return 0 when the clock rate cannot
be determined properly by the recalc_rate hook. Mention in the
documentation that the framework is ok with that.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220816112530.1837489-5-maxime@cerno.tech
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>