soc: samsung: Use kbasename instead of open coding

In preparation to not store the full path of nodes in full_name, use
kbasename instead as it will work either with the full path or not.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Kukjin Kim <kgene@kernel.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: linux-samsung-soc@vger.kernel.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>