PM / Domains: Remove obsolete "samsung,power-domain" check

Currently the generic PM Domain code code checks for the presence of
both (generic) "power-domains" and (Samsung Exynos legacy)
"samsung,power-domain" properties in all device tree nodes representing
devices.

There are two issues with this:
  1. This imposes a small boot-time penalty on all platforms using DT,
  2. Platform-specific checks do not really belong in core framework
     code.

Remove the platform-specific check, as the last user of
"samsung,power-domain" was removed in commit 46dcf0ff ("ARM:
dts: exynos: Remove exynos4415.dtsi").  All other users were converted
before in commit 0da65870 ("ARM: dts: convert to generic power
domain bindings for exynos DT").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>