hw/arm/stellaris: Convert SSYS to QOM device

hw/arm/stellaris: Convert SSYS to QOM device

Convert the SSYS code in the Stellaris boards (which encapsulates the
system registers) to a proper QOM device.  This will provide us with
somewhere to put the output Clock whose frequency depends on the
setting of the PLL configuration registers.

This is a migration compatibility break for lm3s811evb, lm3s6965evb.

We use 3-phase reset here because the Clock will need to propagate
its value in the hold phase.

For the moment we reset the device during the board creation so that
the system_clock_scale global gets set; this will be removed in a
subsequent commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210128114145.20536-17-peter.maydell@linaro.org
Message-id: 20210121190622.22000-17-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>