hw/arm: Remove STRONGARM->PXA2XX dependency

hw/arm: Remove STRONGARM->PXA2XX dependency

Currently the STRONGARM KConfig symbol pulls in PXA2XX. Since we've now
removed all the true uses of PXA2XX, we'd like to remove the PXA2XX
symbol too. To permit that, make STRONGARM directly select the things
it truly depends on:
 * pxa25x-timer
 * SSI

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: 20240903160751.4100218-14-peter.maydell@linaro.org