aarch64-softmmu.mak: Use an ARM specific config

In preperation for having an ARM and MicroBlaze ZynqMP machine let's
split out the current ARM specific config options.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>