dt-bindings: Add #defines for ZTE ZX296702 clocks

Add clocks defines for the global clock controller
found on ZTE ZX296702 SoCs.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Kevin Hilman <khilman@linaro.org>