ARM: dts: sbc-t3x: refactor DT support

Refactor the sbc-t3x device tree as a preparation for additional
(sbc-t3530, sbc-t3517, etc.) boards support.

No functional changes.

The device tree will have the following structure:

omap3-cm-t3x.dtsi
 |
 |<-- omap3-cm-t3x30.dtsi
 |     |
 |     |
 |     |     -----                  -------          ------------
 |     |    | CoM |                | Board |        | Base board |
 |     |     -----                  -------          ------------
 |     |                                            omap3-sb-t35.dtsi
 |     |                                                  |
 |     |<-- omap3-cm-t3730.dts <-- omap3-sbc-t3730.dts -->|
 |     |                                                  |
 |     |<-- omap3-cm-t3530.dts <-- omap3-sbc-t3530.dts -->|
 |                                                        |
 |<-------- omap3-cm-t3517.dts <-- omap3-sbc-t3517.dts -->|

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>