tests/avocado: Truncate M2S-FG484 SOM SPI flash to 16MiB

tests/avocado: Truncate M2S-FG484 SOM SPI flash to 16MiB

The M2S-FG484 SOM uses a 16 MiB SPI flash (Spansion
S25FL128SDPBHICO).  Since the test asset is bigger,
truncate it to the correct size to avoid when running
the test_arm_emcraft_sf2 test:

  qemu-system-arm: device requires 16777216 bytes, block backend provides 67108864 bytes

Add comment regarding the M2S-FG484 SOM hardware in
hw/arm/msf2-som.c.

Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>