dt-bindings: EXYNOS: Add Exynos5433 PMU compatible

This patch documents the usage of Exynos5433 PMU (Power Management Unit)
compatible.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>