target/riscv: Tell gdbstub the correct number of CSRs

target/riscv: Tell gdbstub the correct number of CSRs

If the number of registers reported to the gdbstub code does not match the
number in the associated XML file, then the register numbers used by the stub
may get out of sync with a remote GDB instance.

Signed-off-by: Jonathan Behrens <jonathan@fintelia.io>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>