dt-bindings: clock: samsung: add bindings for Exynos external clock

Document the bindings for Samsung Exynos external to SoC
(oscclk/XXTI/XusbXTI) clock provided on boards.  The bindings are
already implemented in most of the Exynos clock drivers and DTS files.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210825134056.219884-3-krzysztof.kozlowski@canonical.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>