usb: cdns3: Add USBSSP platform driver support

The Cadence USBSSP (CDNSP) controller was previously only accessible
through PCI, coupling the gadget driver with the PCI glue layer into a
single monolithic module (cdnsp-udc-pci). This prevented using the
CDNSP IP on SoC/platform designs that expose the controller through
device tree. It restructures the driver to decouple the CDNSP gadget
from PCI.

- Introduce CONFIG_USB_CDNSP as a standalone tristate (analogous to
  CONFIG_USB_CDNS3), with USB_CDNSP_GADGET and USB_CDNSP_HOST as
  bool sub-options. The gadget code builds as a separate cdnsp.ko
  module.

- Regroup USBSSP and CDNS3 Kconfig options under the USB_CDNS_SUPPORT
  menu so they appear properly grouped in menuconfig.

- Refactor cdnsp-pci.c into a thin PCI-to-platform wrapper (similar
  to cdns3-pci-wrap.c) that registers a platform device and passes
  PCI resources and platform data to the common platform driver.

- Auto-detect the controller version (USBSS vs USBSSP) at runtime by
  reading the DRD/OTG Device ID register in cdns_drd_init(), and select
  the appropriate gadget init function (cdns3_gadget_init or
  cdnsp_gadget_init) based on cdns->version. This follows the same
  pattern already used for host initialization.

- Fix gadget-export.h to use IS_REACHABLE() keyed on the tristate
  module config (CONFIG_USB_CDNS3/CONFIG_USB_CDNSP) instead of
  IS_ENABLED() on the bool gadget config. The bool configs are always
  'y' when enabled, causing IS_ENABLED/IS_REACHABLE to always return
  true and resulting in link errors when cdns-usb-common is built-in
  but the gadget module is loadable.

- Add missing MODULE_LICENSE()/MODULE_DESCRIPTION() and
  EXPORT_SYMBOL_GPL() to the cdns3 and cdnsp gadget modules, required
  by modpost.

- Pass override_apb_timeout through cdns3_platform_data so the PCI
  wrapper can communicate PCI-specific APB timeout values to the
  common driver.

This patch is Assisted-by: Cursor:claude-4.6-opus

Signed-off-by: Peter Chen <peter.chen@cixtech.com>
Acked-by: Pawel Laszczak <pawell@cadence.com>
Link: https://patch.msgid.link/20260316064831.274865-3-peter.chen@cixtech.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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