clk: shmobile: Fix typo in MSTP clock DT bindings

The DT bindings document a renesas,indices property, while the code, the
DT example and the DT sources all use renesas,clock-indices. Fix the
documentation.

The shmobile mstp DT bindings have been merged in v3.14-rc1 with a bug
in the DT ABI, a fix during the -rc series is appropriate.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms@verge.net.au>