arm64: dts: ti: Makefile: Collate AM64 platforms together

Make sure that the platforms are grouped together per SoC. This helps
keep the Makefile readable as newer platforms get added to the list.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20210915121442.27112-1-nm@ti.com