hw/arm/exynos: Use type define instead of hard-coded a9mpcore_priv string

hw/arm/exynos: Use type define instead of hard-coded a9mpcore_priv string

Use a define for a9mpcore_priv device type name instead of hard-coded
string.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>