arm64: dts: exynos: Describe PWM interrupts on Exynos7

Add interrupts property to PWM node on Exynos7 to describe the hardware
fully.  No functional change as the interrupts are not used by drivers.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>