usb: cdns3: Split core.c into cdns3-plat and core.c file

Patch splits file core.c into core.c containing the common reusable code
and cnd3-plat.c containing device platform specific code. These changes
are required to make possible reuse DRD part of CDNS3 driver in CDNSP
driver.

Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Tested-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>