PCI: ultrarisc: Use module_platform_driver()

CONFIG_PCIE_ULTRARISC is a tristate option and may be built as a module.
Use module_platform_driver() so the driver uses the standard registration
helper for both built-in and module configurations.

Leave .remove() unset because the driver registers an internal MSI
controller and the IRQs cannot be safely disposed during removal.

Signed-off-by: Jia Wang <wangjia@ultrarisc.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260731-ultrarisc-pci-clk-v4-3-d738b491e49a@ultrarisc.com