dt-bindings: soc: samsung: exynos-pmu: use abolute ref paths

Preferred coding for referencing other schemas is to use absolute path.
Quotes over path are also not needed.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220706160257.27579-2-krzysztof.kozlowski@linaro.org