riscv: Pass Object to register_cpu_props instead of DeviceState

riscv: Pass Object to register_cpu_props instead of DeviceState

One can extract the DeviceState pointer from the Object pointer, so pass
the Object for future commits to access other fields of Object.

No functional changes intended.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-ID: <20230303131252.892893-2-alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>