dts/bindings: move RISC-V cores bindings to `dts/bindings/cpu/`

This commit moves the bindings of RISC-V cores from `dts/bindings/riscv` to
`dts/bindings/cpu`. This change aligns the bindings of RISC-V cores with
other architectures.

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