The event channel is established statically for communication between
two domains. There is no need to define event channels in device tree
if only one domain is running on Xen. Also, since the event channel
must describe the foreign-evtchn phandle connected to in the device
tree, this is missing in the case of only one domain running on Xen,
which causes the device tree overlay to fail.

Based on the above reason, this patch adds a switch to enable the
description of evtchn in the device tree. Whether it is enabled or not
is determined by the 'XEN_DOM0LESS_DOMAINS' configuration: if only one
domain runs on Xen, evtchn will be excluded from the device tree.

This functionality is achieved by:
1. Wrap the evtchn node in '#if VAR / #endif'
2. The value of VAR (0 or 1) is determined by XEN_DOM0LESS_DOMAINS
3. Use cpp (c preprocessor) to generate dts that meet the conditions

Issue-Id: SCM-5451
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I9c96f0b9de502757a1f29c6f022addb842990bee