ARM: EXYNOS: Fix potential NULL pointer access in exynos_sys_powerdown_conf

If no platform devices binded to the driver but driver itself loaded and
exynos_sys_powerdown_conf is called from
arch/arm/mach-exynos/{suspend.c, pm.c} it will result in NULL pointer access,
to prevent this added check on pmu_context for NULL.

Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Tested-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>