arm64: dts: ti: Use OF_ALL_DTBS for combined blobs

Combined dtb builds are only useful for making sure that the overlay
applies cleanly on the base dtb.

So we move all such combined blobs under a `dtb- +=` section that is
only built when CONFIG_OF_ALL_DTBS is enabled.

Signed-off-by: Jai Luthra <j-luthra@ti.com>
Link: https://lore.kernel.org/r/20231201-csi_dts-v3-9-9f06f31080fe@ti.com


Signed-off-by: Nishanth Menon <nm@ti.com>