soc: samsung: Do not build ARMv7 PMU drivers on ARMv8

The Exynos Power Management Unit (PMU) drivers contain quite large
static arrays of register values necessary for given Exynos SoC to enter
low power mode.  All this data is useless for ARMv8 SoC like
Exynos5433, because the image will not be shared between ARMv7 and
ARMv8.

Add additional Kconfig symbol for selecting the SoC-specific driver
addons thus skipping the useless data in the final image (this is
similar approach to chosen for Exynos clock controller drivers):
 - exynos-pmu driver will be compiled on both architectures ARMv7
   and ARMv8,
 - additional driver_data for ARMv7 SoCs will not be built on ARMv8
   and a macro will return NULL for them in of_device_id - this should
   be safe as these compatibles cannot match on ARMv7 and driver
   anyway handles NULL driver_data,
 - on ARMv8 compile only exynos-pmu driver which exposes the
   syscon-regmap for PMU address space.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>