hw/arm/pxa2xx: Simplify pxa255_init()

Since pxa255_init() must map the device in the system memory,
there is no point in passing get_system_memory() by argument.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230109115316.2235-2-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>