target/riscv: Implement optional CSR mcontext of debug Sdtrig extension

target/riscv: Implement optional CSR mcontext of debug Sdtrig extension

The debug Sdtrig extension defines an CSR "mcontext". This commit
implements its predicate and read/write operations into CSR table.
Its value is reset as 0 when the trigger module is reset.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231219123244.290935-1-alvinga@andestech.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>