ARM: dts: imx25: Fix PWM "per" clocks

Currently PWM functionality is broken on mx25 due to the wrong assignment of the
PWM "per" clock.

According to Documentation/devicetree/bindings/clock/imx25-clock.txt:
	pwm_ipg_per		52

,so update the pwm "per" to use 'pwm_ipg_per' instead of 'per10' clock.

With this change PWM can work fine on mx25.

Cc: <stable@vger.kernel.org>
Reported-by: Carlos Soto <csotoalonso@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>