ramips: use full 8MB flash on ZyXEL Keenetic

ZyXEL Keenetic has 8MB flash, but OpenWrt uses only 4MB.
This commit fixes the problem.

WikiDevi page [1] says that ZyXEL Keenetic has FLA1: 8 MiB, there is
an article with specs [2] (in Russian).

[1] https://wikidevi.wi-cat.ru/ZyXEL_Keenetic
[2] https://3dnews.ru/608774/page-2.html

Fixes: FS#2487
Fixes: a7cbf59e0e ("ramips: add new device ZyXEL Keenetic as kn")

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>
(cherry picked from commit fea232ae8f)

target/linux/ramips/dts/kn.dts

@@ -48,7 +48,7 @@
 			partition@50000 {
 				compatible = "denx,uimage";
 				label = "firmware";
-				reg = <0x50000 0x3b0000>;
+				reg = <0x50000 0x7b0000>;
 			};
 		};
 	};

target/linux/ramips/image/rt305x.mk

@@ -905,7 +905,7 @@ TARGET_DEVICES += xdxrn502j
 define Device/kn
   DTS := kn
   BLOCKSIZE := 64k
-  IMAGE_SIZE := $(ralink_default_fw_size_4M)
+  IMAGE_SIZE := 7872k
   DEVICE_TITLE := ZyXEL Keenetic
   DEVICE_PACKAGES := kmod-usb-core kmod-usb2 kmod-usb-ehci kmod-usb-ledtrig-usbport
   DEFAULT := n