dt-bindings: pwm: mxs: Document fsl,imx28-pwm

imx28 uses the same PWM block that is found on imx23.

Add an entry for fsl,imx28-pwm.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>