clk: add clock-indices support

Add a property called clock-indices to allow clock-output-names
to be used where the index used to lookup a clock is not a 1:1
mapping to the array position in the clock-output-names

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
(cherry picked from commit 7a0fc1a3df82d29e00b4c9f88a6b37450d6711f1)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>