ARM: dts: exynos: Cleanup power domain nodes in exynos3250.dtsi

Device nodes in device tree should use generic names, so rename
all existing power domains to "power-domain". To keep readable domain
names in debug logs, use label property, which has been introduce by
commit b13b2330 ("soc: samsung: pm_domains: Read domain name from
the new label property");

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>