Add MSTP clock support for the r8a7779 SoC

The only change required appears to be to note a new
compatibility string in the bindings.

Cc: devicetree@vger.kernel.org
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mike Turquette <mturquette@linaro.org>