serial: samsung: Keep a copy of the location of platform data in driver's private data

Add a pointer to the location of the platform data in the driver's private
data. When instantiated using device tree, pdev->dev->platform_data does not
necessarily point to a valid instance of platform data. The platform data
pointer in the driver's private data could be set to pdev->dev->platform_data
or platform data instance created from device tree.

Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>