clk: Improve clock-indices binding documentation

Improve the wording for the clock-indices binding documentation.
Also replace "empty nodes" by "empty strings", as reported before by Sergei
Shtylyov.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Mike Turquette <mturquette@linaro.org>