xen/arm: domain_builder: irq sanity check logic fix

xen/arm: domain_builder: irq sanity check logic fix

Since commit "xen/arm: domain_build: Rework the way to allocate the
event channel interrupt", it is not possible for an irq to be both below 16
and greater/equal than 32.

Also fix the reference to linux documentation while we're at it.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
[Slightly rework the commit message]