ARM: shmobile: r8a7779: Add clock index macros for DT sources

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>