rust: make `clk::Hertz` methods const

rust: make `clk::Hertz` methods const

Marks `Hertz` methods as `const` to make them available
for `const` contexts. This can be useful when defining
static/compile-time frequency parameters in drivers/subsystems.

Signed-off-by: Onur Özkan <work@onurozkan.dev>
Link: https://lore.kernel.org/r/20250618091442.29104-1-work@onurozkan.dev
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>