clk: qcom: Add APQ8084 Global Clock Controller documentation

Add the compatible string for the APQ8084 global clock controller
to the clock binding documentation.

Signed-off-by: Georgi Djakov <gdjakov@mm-sol.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>