xen/arm: domain_build: Rework the way to allocate the event channel interrupt

xen/arm: domain_build: Rework the way to allocate the event channel interrupt

At the moment, a placeholder will be created in the device-tree for the
event channel information. Later in the domain construction, the
interrupt for the event channel upcall will be allocated the device-tree
fixed up.

Looking at the code, the current split is not necessary because all the
PPIs used by the hardware domain will by the time we create the node in
the device-tree.

>From now, mandate that all interrupts are registered before
acpi_prepare() and dtb_prepare(). This allows us to rework the event
channel code and remove one placeholder.

Note, this will also help to fix the BUG(...) condition in set_interrupt_ppi
which is completely wrong. See in a follow-up patch.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>