dt-bindings: samsung: pmu: Document Exynos850

Exynos850 SoC can reuse PMU driver functionality. Add corresponding
compatible string to PMU bindings documentation.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211028144313.9444-1-semen.protsenko@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>