dt-bindings: clock: Add bindings for SP7021 clock driver

Add documentation to describe Sunplus SP7021 clock driver bindings.

Signed-off-by: Qin Jian <qinjian@cqplus1.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>