ARM: dts: imx6sll: add PXP module

While the EPDC is optional, both consumer and industrial editions
have the PXP module, so adding it to the corresponding .dtsi
Information taken from freescale kernel, compared with the
reference manual and tested by a separate program.

Since it does not depend on external wiring, the
status = "disabled" is left out here.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>