dt-bindings: soc: samsung: keep SoC driver bindings together

Recently added Samsung Exynos USI driver devicetree bindings were added
under ../bindings/soc/samsung/exynos-usi.yaml, so move there also two
other bindings for Exynos SoC drivers: the PMU and ChipID.

Update Samsung Exynos MAINTAINERS entry to include this new path.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20211213112057.16709-1-krzysztof.kozlowski@canonical.com