sunxi: update a13-olimex-som DTS path

In preparation for supporting kernel 6.6, where the DTS files are grouped into
vendors - similarly to what arm64 has been doing all along -, update the
SUNXI_DTS var of this board to prepend it with SUNXI_DTS_DIR.

Signed-off-by: Zoltan HERPAI <[email protected]>

target/linux/sunxi/image/cortexa8.mk

@@ -43,7 +43,7 @@ define Device/olimex_a13-olimex-som
   DEVICE_PACKAGES:=kmod-rtl8192cu
   SUPPORTED_DEVICES:=olimex,a13-olinuxino
   SOC := sun5i-a13
-  SUNXI_DTS := sun5i-a13-olinuxino
+  SUNXI_DTS := $$(SUNXI_DTS_DIR)sun5i-a13-olinuxino
 endef
 TARGET_DEVICES += olimex_a13-olimex-som