clk: Clarify clk_get_rate() expectations

clk: Clarify clk_get_rate() expectations

As shown by a number of clock users already, clk_get_rate() can be
called whether or not the clock is enabled.

Similarly, a number of clock drivers will return a rate of 0 whenever
the rate cannot be figured out.

Since it was a bit ambiguous before, let's make it clear in the
clk_get_rate() documentation.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220816112530.1837489-6-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>