i2c: designware: Fix falling time bindings doc

In (6468276b i2c: designware: make SCL and SDA falling time
configurable) new device tree properties were added for setting the
falling time of SDA and SCL.  The device tree bindings doc had a typo
in it: it forgot the "-ns" suffix for both properies in the prose of
the bindings.

I assume this is a typo because:
* The source code includes the "-ns"
* The example in the bindings includes the "-ns".

Fix the typo.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Fixes: 6468276b ("i2c: designware: make SCL and SDA falling time configurable")
Acked-by: Romain Baeriswyl <romain.baeriswyl@alitech.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org