ramips: use upstream RAW_APPENDED_DTB instead of our OWRTDTB

Upstream kernel added support for RAW_APPENDED_DTB on ralink arch
in the following commit:
02564fc89d3d ("ralink: Introduce fw_passed_dtb to arch/mips/ralink")

Use upstream solution and get rid of our OWRTDTB hack.
This commit set DEVICE_DTS to $$(DTS) instead of replacing DTS with
DEVICE_DTS in device profile because DTS variable will be dropped
in later commits.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
[Tested on mt7621/mt76x8]
Tested-by: Chuanhong Guo <gch981213@gmail.com>
[Tested on rt305x/mt7620]
Tested-by: INAGAKI Hiroshi <musashino.open@gmail.com>