ARM: dts: imx6qdl: Allow disabling the PU regulator, add a enable ramp delay

The PU regulator is enabled during boot, but not necessarily always-on.
It can be disabled by the generic pm domain framework when the PU power
domain is shut down. The ramp delay of 150 us might be a bit conservative,
the value is taken from the Freescale kernel.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>