i2c: sun6i-p2wi: make bindings file name match the driver

If we use the "i2c-" prefix for the binding documentation file name,
then it should match the file name of the driver, if possible. It is
possible for this driver, so rename it.
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>