hw/misc/mps2-fpgaio: Use the LED device

hw/misc/mps2-fpgaio: Use the LED device

Per the 'ARM MPS2 and MPS2+ FPGA Prototyping Boards Technical
Reference Manual' (100112_0200_07_en):

  2.1  Overview of the MPS2 and MPS2+ hardware

       The MPS2 and MPS2+ FPGA Prototyping Boards contain the
       following components and interfaces:

       * User switches and user LEDs:

         - Two green LEDs and two push buttons that connect to
           the FPGA.
         - Eight green LEDs and one 8-way dip switch that connect
           to the MCC.

Add the 2 LEDs connected to the FPGA.

This replaces the 'mps2_fpgaio_leds' trace events by the generic
'led_set_intensity' event.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc.michel@greensocs.com>
Message-Id: <20200912134041.946260-6-f4bug@amsat.org>