clk: shmobile: clk-mstp: change to using clock-indices

With the addition of clock-indices, we need to change the renesas
clock implementation to use these instead of the local definition
of "renesas,clock-indices".

Since this will break booting with older device trees, we add a
simple auto-detection of which properties are present.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>