ARM: dts: exynos: enable PMIC wakeup from suspend on SMDK5420

The RTC on S2MPS11 PMIC can wakeup the system from suspend to RAM.
Add a generic property for this.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210420164943.11152-9-krzysztof.kozlowski@canonical.com