dt-bindings: soc: samsung: Add Exynos PMU

Add bindings documentation for Exynos PMU hardware block. It was taken
from Linux kernel [1], but minimized and modified to reflect features
that will be actually supported in U-Boot soon. For example,
the "samsung,uart-debug-1" property is not available in Linux kernel
bindings and only present in U-Boot.

[1] Documentation/devicetree/bindings/soc/samsung/exynos-pmu.yaml

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>