clk_round_rate() can be used by drivers to determine whether or not a
frequency is supported by the clock. The current Tegra clock driver
outputs an error message and a stacktrace when the requested rate isn't
supported. That's fine for clk_set_rate(), but it's confusing when all
the driver does is query whether or not a frequency is supported.

Signed-off-by: Thierry Reding <treding@nvidia.com>