riscv: dts: Add dma-channels property and modify compatible

Add dma-channels property, then we can determine how many channels there
by device tree, in addition, we add the pdma versioning scheme for
compatible.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>