irqdomain: Move revmap_trees_mutex to struct irq_domain

The revmap_trees_mutex protects domain->revmap_tree.  There is no
need to make it global because it is allowed to modify revmap_tree
of two different domains concurrently.  Having said that, this would
not be a actual bottleneck because the interrupt map/unmap does not
occur quite often.  Rather, the motivation is to tidy up the code
from a data structure point of view.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>