hw/misc/mps2-fpgaio: Support AN547 DBGCTRL register

hw/misc/mps2-fpgaio: Support AN547 DBGCTRL register

For the AN547 image, the FPGAIO block has an extra DBGCTRL register,
which is used to control the SPNIDEN, SPIDEN, NPIDEN and DBGEN inputs
to the CPU.  These signals control when the CPU permits use of the
external debug interface.  Our CPU models don't implement the
external debug interface, so we model the register as
reads-as-written.

Implement the register, with a property defining whether it is
present, and allow mps2-tz boards to specify that it is present.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210219144617.4782-39-peter.maydell@linaro.org