arm: dts: ti: am57x-idk: Enable support for building pps dtb

commit fe7d0006 ("arm: dts: ti: dt-overlay: Add a pps overlay
for am57xx idk platforms") added support for building overlays. But
missed creating dtbs using dtb-merge.cfg

Update Makefile and dtb-merge.cfg to build all am57x-idk dtbs specific
to pps.

Fixes: fe7d0006 ("arm: dts: ti: dt-overlay: Add a pps overlay for am57xx idk platforms")
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>