ARM: imx53: qsrb: fix PMIC interrupt level

The MC34708 PMIC interrupt level is active high, but was set to
active low in the devicetree, probably as a result of a copy and
paste error from the QSB board.

This caused IRQ storms and led to the kernel disabling the PMIC
interrupt.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>