soc: samsung: pm_domains: Prepare for supporting ARMv8 Exynos

The ARMv8 Exynos family (Exynos5433 and Exynos7420) uses different value
(0xf instead of 0x7) for controlling the power domain on/off registers
(both for control and for status).

Choose the value depending on the compatible. This prepares the driver
for supporting ARMv8 SoCs.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>