hw/input/stellaris_gamepad: Remove StellarisGamepadButton struct

hw/input/stellaris_gamepad: Remove StellarisGamepadButton struct

Currently for each button on the device we have a
StellarisGamepadButton struct which has the irq, keycode and pressed
state for it.  When we convert to qdev, the qdev property and GPIO
APIs are going to require that we have separate arrays for the irqs
and keycodes.  Convert from array-of-structs to three separate arrays
in preparation.

This is a migration compatibility break for the stellaris boards
(lm3s6965evb, lm3s811evb).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20231030114802.3671871-5-peter.maydell@linaro.org
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v1=>v2: mention migration compat break in commit message;
  bump version fields in vmstate