ARM: EXYNOS: Use platform device name as power domain name

The power domain nodes in DTS may be very generic (e.g. "power-domain"
for Exynos 5420) making it very hard to debug:

$ cat /sys/kernel/debug/pm_genpd/pm_genpd_summary
    domain                      status         slaves
power-domain                    on

Use platform device name instead so the names will be a little more
user friendly:
    domain                      status         slaves
100440e0.power-domain           on

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Suggested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Suggested-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>