Add macros usable by device tree sources to reference r8a7779 clocks by
index.

Based on work for the r8a7791 SoC by Laurent Pinchart.
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>