dts/riscv: remove the `timebase-frequency` property

The `timebase-frequency` is not defined by any of the YAML binding files.
There was a discussion in #37420 to add this property, but in the end it
was rejected. This resulted in the #37685 feature request being created.

As of now, this property is not documented anywhere so this commit removes
it from the RISC-V devicetrees, as RISC-V is the only architecture that is
currently defining it - and even in RISC-V not all platforms do that.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>