dt-bindings: soc: samsung: Add Exynos USI bindings

Add constants for choosing USIv2 configuration mode in device tree.
Those are further used in USI driver to figure out which value to write
into SW_CONF register. Also document USIv2 IP-core bindings.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211204195757.8600-2-semen.protsenko@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>