ARM: dts: overo: reorganize include files

Currently, overo-related include files are organized as follow:

                     omap3-overo.dtsi
                             |
                             |
omap34xx.dtsi   omap3-overo-tobi-common.dtsi   omap36xx.dtsi
      |              |                 |             |
      ---------------                  ---------------
              |                                |
    omap3-overo-tobi.dts          omap3-overo-storm-tobi.dts

This is unpractical when one has to deal with SoC-specific pinmux
belonging to the omap3_pmx_core2 (defined in omap34xx/omap36xx),
for pins related to the processor board. With the current
hierarchy, such pinmux has to be defined in the expansion board's
.dts, which is not logical.

This patches reorganizes the files to add (yet another) abstraction
layer between the processor and the expansion boards.

omap34xx.dtsi       omap3-overo-base.dtsi      omap36xx.dtsi
      |              |                 |             |
      ---------------                  ---------------
              |                                |
      omap3-overo.dtsi               omap3-overo-storm.dtsi
              |                                |
      --------                                  ------
      |         omap3-overo-tobi-common.dtsi         |
      |              |                 |             |
      ---------------                  ---------------
              |                                |
    omap3-overo-tobi.dts          omap3-overo-storm-tobi.dts

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>