ARM: shmobile: r8a7791 dtsi: Change to using clock-indices

With the addition of clock-indices in commit 8e33f91a

 ("clk:
shmobile: clk-mstp: change to using clock-indices"), we can change the
DTSes to use the generic property instead of the deprecated
vendor-specific property.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[geert: Extracted r8a7791-specific part, rebased, reworded]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>