xen/arm: configure dom0less domain for enabling xenstore after boot

xen/arm: configure dom0less domain for enabling xenstore after boot

Export evtchn_alloc_unbound and make it __must_check.

If "xen,enhanced" is enabled, then add to dom0less domains:

- the hypervisor node in device tree
- the xenstore event channel

The xenstore event channel is also used for the first notification to
let the guest know that xenstore has become available.

Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
CC: Julien Grall <julien@xen.org>
CC: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
CC: Bertrand Marquis <bertrand.marquis@arm.com>