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>

include/linux/clk-provider.h

@@ -118,8 +118,9 @@ struct clk_duty {
  *
  * @recalc_rate	Recalculate the rate of this clock, by querying hardware. The
  *		parent rate is an input parameter.  It is up to the caller to
- *		ensure that the prepare_mutex is held across this call.
- *		Returns the calculated rate.  Optional, but recommended - if
+ *		ensure that the prepare_mutex is held across this call. If the
+ *		driver cannot figure out a rate for this clock, it must return
+ *		0. Returns the calculated rate. Optional, but recommended - if
  *		this op is not set then clock rate will be initialized to 0.
  *
  * @round_rate:	Given a target rate as input, returns the closest rate actually