ARM: dts: Add new "generic" am4372 device tree file.

With U-boot runtime board detect for DTB selection a "default" dtb needs
to be created. This will be used temporarily until the "proper" dtb is
selected.

Also, add -u-boot.dtsi for AM437x SK and IDK to enable I2C for
board detection via DM_I2C.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>