powerpc/dts: don't fall back to fsl,pq3-gpio for fsl,mpc8572-gpio

While the handling of fsl,pq3-gpio and fsl,mpc8572-gpio is done in the
same driver and the two hardly differ, the latter controller needs a
workaround for an erratum in the gpio_get callback. To make this
difference more explicit remove fsl,pq3-gpio from the list of
compatibles for mpc8572 machines.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>