dt-bindings: i2c: pxa: Update the documentation for the Armada 3700

This commit documents the compatible string to have the compatibility for
the I2C unit found in the Armada 3700.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>