xen/arm: Enable the existing x86 virtual PCI support for ARM

xen/arm: Enable the existing x86 virtual PCI support for ARM

The existing VPCI support available for X86 is adapted for Arm.
When the device is added to XEN via the hyper call
“PHYSDEVOP_pci_device_add”, VPCI handler for the config space
access is added to the Xen to emulate the PCI devices config space.

A MMIO trap handler for the PCI ECAM space is registered in XEN
so that when guest is trying to access the PCI config space,XEN
will trap the access and emulate read/write using the VPCI and
not the real PCI hardware.

For Dom0less systems scan_pci_devices() would be used to discover the
PCI device in XEN and VPCI handler will be added during XEN boots.

This patch is also doing some small fixes to fix compilation errors on
arm32 of vpci and prevent 64bit accesses on 32bit:
- use %zu instead of lu in header.c for print
- prevent 64bit accesses in vpci_access_allowed
- ifdef out using CONFIG_64BIT handling of len 8 in
vpci_ecam_{read/write}

TODO: currently vpci_add_handlers is marked as __hwdom_init, but on ARM
vpci_add_handlers can be called after boot from
PHYSDEVOP_pci_device_add. Consider removing __hwdom_init.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
[stefano: add TODO item to commit message]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>