usb: cdns3: Add USBSSP platform driver support

Expose Cadence USBSSP through the same platform path as USBSS, trim
Kconfig and Makefile: one core loadable object plus separate glue .ko
files.

Single cdns.ko bundles core, DRD, the generic "cdns,usb3" platform
driver in cdns3-plat.c, optional host.o, and optional gadget objects.
Use CONFIG_USB_CDNS3_GADGET as a bool to compile gadget support into
that module. Remove duplicate MODULE_* declarations from cdns3-plat.c
now that it links into the same module.

Kconfig: the generic platform driver is selected via CONFIG_USB_CDNS3.
Move CONFIG_USB_CDNSP_PCI beside CONFIG_USB_CDNS3_PCI_WRAP under
"Platform glue driver support". SoC glue entries (TI, i.MX, StarFive)
depend only on CONFIG_USB_CDNS3.

Tighten CONFIG_USB_CDNS_SUPPORT dependencies so the umbrella follows
host or gadget when either is built as a module. Match host and gadget
bools to the cdns.ko tristate with USB=USB_CDNS3 and USB_GADGET=USB_CDNS3
instead of comparing against USB_CDNS_SUPPORT.

Link host.o when CONFIG_USB_CDNS3_HOST is enabled and use that symbol in
host-export.h, removing the redundant CONFIG_USB_CDNS_HOST indirection.

Export cdns_core_init_role and reorganize the function cdns_init, and
controller version could be gotten before the gadget init function is
decided per controller.

Keep host_init / gadget_init callbacks in struct cdns, so core.c does
not need direct linkage to host or gadget objects. Refactor cdnsp-pci.c
into a thin PCI-to-platform wrapper.

drivers/usb/Makefile: descend into drivers/usb/cdns3/ only when
CONFIG_USB_CDNS_SUPPORT is enabled.

Assisted-by: Cursor:claude-4.6-opus
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Chen <peter.chen@cixtech.com>