dt-bindings: clock: rpmcc: Document msm8976 compatible

Support for MSM8976 was added to the clk-smd-rpm driver: let's
document here the newly added compatible string.

Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Link: https://lkml.kernel.org/r/20191031112951.35850-3-kholk11@gmail.com


Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>