dt-bindings: clk: oxnas,stdclk: Add OX820 bindings

Add OX820 bindings and remove clock indices from bindings since they are present
in the dt-bindings headers files.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20161005150752.22618-7-narmstrong@baylibre.com