dt-bindings: clock: Add support for the MSM8998 mmcc

Document the multimedia clock controller found on MSM8998.

Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/1576596018-10140-1-git-send-email-jhugo@codeaurora.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>