dt-bindings: soc: samsung: Add Exynos USI

Add USI bindings documentation and header file. Those are taken from
Linux kernel [1,2], but the documentation was reworked a bit to only
describe the compatibles that will be supported in U-Boot soon.

[1] Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml
[2] include/dt-bindings/soc/samsung,exynos-usi.h

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