ipq806x: move Device DTS to dedicated DTS directory

Align the ipq806x target to the pattern already used on other devices where
the device DTS are placed in a dedicated directory separate from the files
directory.

This, while trying to enforce a common pattern for every target, also permits to
do modification to device DTS without having to trigger a recompilation of the
entire kernel (as the files directory is not touched)

Link: https://github.com/openwrt/openwrt/pull/22039
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

target/linux/ipq806x/image/Makefile

@@ -6,7 +6,7 @@ include $(INCLUDE_DIR)/image.mk
 define Device/Default
 	PROFILES := Default
 	KERNEL_LOADADDR = 0x42208000
-	DEVICE_DTS_DIR = $$(DTS_DIR)/qcom
+	DEVICE_DTS_DIR = ../dts
 	DEVICE_DTS = $$(SOC)-$(lastword $(subst _, ,$(1)))
 	DEVICE_DTS_CONFIG := config@1
 	IMAGES := sysupgrade.bin

target/linux/ipq806x/image/chromium.mk

@@ -1,3 +1,5 @@
+DTS_DIR := $(DTS_DIR)/qcom
+
 define Device/dsa-migration
   DEVICE_COMPAT_VERSION := 1.1
   DEVICE_COMPAT_MESSAGE := Config cannot be migrated from swconfig to DSA

target/linux/ipq806x/image/generic.mk

@@ -1,3 +1,4 @@
+DTS_DIR := $(DTS_DIR)/qcom
 DEVICE_VARS += NETGEAR_BOARD_ID NETGEAR_HW_ID
 DEVICE_VARS += TPLINK_BOARD_ID