msm: pcie: register PCI driver with framework for root port

Register root port with PCI driver framework so that
it will get probe during PCI enumeration. This provides
the root port the option to configure its pci_dev.

Change-Id: Ia52bb2e4230fe9b8d3431022d89125e113463a80
Signed-off-by: Tony Truong <truong@codeaurora.org>