ARM: dts: opos6ul/opos6uldev: rework device tree to support i.MX6ULL

Rework the device trees of the OPOS6UL and OPOS6ULDev boards to support
the OPOS6UL SoM with an i.MX6ULL SoC.  The device trees are now as
following:

- imx6ul-imx6ull-opos6ul.dtsi
  common for both i.MX6UL and i.MX6ULL OPOS6UL SoM.
- imx6ul-opos6ul.dtsi
  for i.MX6UL OPOS6UL SoM. It includes imx6ul.dtsi and
  imx6ul-imx6ull-opos6ul.dtsi.
- imx6ull-opos6ul.dtsi
  for i.MX6ULL OPOS6UL SoM. It includes imx6ull.dtsi and
  imx6ul-imx6ull-opos6ul.dtsi.

- imx6ul-imx6ull-opos6uldev.dtsi
  OPOS6ULDev base device tree.
- imx6ul-opos6uldev.dts
  OPOS6ULDev board with an i.MX6UL OPOS6UL SoM. It includes
  imx6ul-opos6ul.dtsi and imx6ul-imx6ull-opos6uldevdtsi.
- imx6ull-opos6uldev.dts
  OPOS6ULDev board with an i.MX6ULL OPOS6UL SoM. It includes
  imx6ull-opos6ul.dtsi and imx6ul-imx6ull-opos6uldevdtsi.

Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>