target/riscv: Add support for mconfigptr

target/riscv: Add support for mconfigptr

RISC-V privileged specification v1.12 introduced a mconfigptr
which will hold the physical address of a configuration data
structure. As Qemu doesn't have a configuration data structure,
is read as zero which is valid as per the priv spec.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220303185440.512391-5-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>