Rework irq_find_mapping() to return an both an irq_desc pointer,
optionally the virtual irq number, and rename the result to
__irq_resolve_mapping(). a new helper called irq_resolve_mapping()
is provided for code that doesn't need the virtual irq number.

irq_find_mapping() is also rewritten in terms of __irq_resolve_mapping().
Signed-off-by: Marc Zyngier <maz@kernel.org>