hw/mips/malta: Trace FPGA LEDs/ASCII display updates

hw/mips/malta: Trace FPGA LEDs/ASCII display updates

The FPGA LEDs/ASCII display is mostly used by the bootloader
to show very low-level debug info. QEMU connects its output
to a character device backend, which is not very practical
to correlate with ASM instruction executed, interrupts or
MMIO accesses. Also, the display discard the previous states.

To ease bootloader debugging experience, add a pair of trace
events. Such events can be analyzed over time or diff-ed
between different runs.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230104133935.4639-4-philmd@linaro.org>