arm64: zynqmp: Rename dt overlay file names from dts to dtso

Use dtso suffix instead of dts. Build option was introduced by
commit a0f9a779 ("kbuild: Allow DTB overlays to built from .dtso named
source files").

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/1dce37e72428c14a3ccbb5dc674b90dfe56b75ac.1695046155.git.michal.simek@amd.com