usb: dwc3: exynos: Use of_platform API to create dwc3 core pdev

Used of_platform_populate() to create dwc3 core platform_device
from device tree data. Additionally some cleanup is also done.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
CC: Felipe Balbi <balbi@ti.com>
CC: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>