PCI: iproc: Fix ARM64 dependency in Kconfig

Allow Broadcom iProc PCIe core driver to be compiled for ARM64

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Vikram Prakash <vikramp@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Olof Johansson <olof@lixom.net>