docs: rust: riscv is maintained

We run it in the CI now (`riscv64` only for the moment).

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

Documentation/rust/arch-support.rst

@@ -27,6 +27,9 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
    * - ``powerpc``
      - Maintained
      - ``ppc64le`` only, ``RUST_OPT_LEVEL < 2`` requires ``CONFIG_THREAD_SHIFT=15``
+   * - ``riscv``
+     - Maintained
+     - ``riscv64`` only
    * - ``x86``
      - Maintained
      - ``x86_64`` only