dt-bindings: update risc-v cpu properties

The Canaan Kendryte K210 SoC CPU cores are based on a rocket chip
version using a draft verion of the RISC-V ISA specifications. To avoid
any confusion with CPU cores using stable specifications, add the
compatible string "canaan,k210" for this SoC CPU cores.

Also add the "riscv,none" value to the mmu-type property to allow a DT
to indicate that the CPU being described does not have an MMU or that
it has an MMU that is not usable (which is the case for the K210 SoC).

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>