soc: samsung: pm_domains: Read domain name from the new label property

Device tree nodes for each power domain should use generic "power-domain"
name, so using it as a domain name doesn't give much benefits. This patch
adds support for human readable names defined in 'label' property. Such
names are visible to userspace and makes debugging much easier. When no
'label' property is found, driver keeps using the name constructed from
full node name.

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