ARM: dts: imx28-tx28: Remove 'enable-active-low' property

'enable-active-low' is not a valid property for a GPIO controlled regulator.

According to Documentation/devicetree/bindings/regulator/gpio-regulator.txt:

"Optional properties:
...
- enable-active-high	: Polarity of GPIO is active high (default is low)."

,so the correct way to define an active-low GPIO controlled regulator is to
simply not pass 'enable-active-high'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>