ARM: dts: Add dtsi files for AM3703, AM3715 and DM3725

DM3730 is considered as omap36xx.dtsi, while the rest has:
     DM3730 | DM3725 | AM3715 | AM3703
IVA    X    |   X    |        |
SGX    X    |        |   X    |
Where X is "supported"

Signed-off-by: André Hentschel <nerv@dawncrow.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>