ARM: EXYNOS: Fix potentian kfree() of ro memory

The change fixes a bug introduced by 2be2a3ff, memory allocated
by kstrdup_const() must be always deallocated with kfree_const(),
otherwise there is a risk of kfree'ing ro memory in power domain error
exit path.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: <stable@vger.kernel.org>
Fixes: 2be2a3ff ("ARM: EXYNOS: register power domain driver from core_initcall")
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>