ARM: shmobile: r8a7791: Add clock index macros for DT sources

Add macros usable by device tree sources to reference r8a7791 clocks by
index.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>