DT: exynos: update PMU binding

Document the fact that some Exynos PMUs are capable of acting as
an interrupt controller.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1426088693-15724-3-git-send-email-marc.zyngier@arm.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>