clk: oxnas: Add dt-bindings include file for OX810SE

In order to prepare support for the Oxford Semiconductor OX820, add
a dt-bindings include file used by the ox810se dtsi.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20161005150752.22618-2-narmstrong@baylibre.com