The power button is not part of the CPU module and can be triggered
wrongly on other baseboards. Disable it since we do not need it
currently.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

The TX6 board come with either NAND flash or eMMC as primary
storage medium. This adds support for the eMMC variants.
We can detect if we have NAND or eMMC by looking at the
bootsource which will be configured accordingly. This
way we can modify the device tree during runtime and do
not have to create a new image.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

The stuff we currently have in the i.MX6q dts file can be reused for
the i.MX6dl variants, so factor out a common dtsi file.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>