ramips: increase HiWiFi HC5962 kernel partition to 4M

Increase kernel partition because 2M is insufficient for 5.4
Because the partition changes, previous version of OpenWrt cannot upgrade
to this version, and requires a new installation

Recovery to stock instruction:
1. Download stock firmware at
   http://ur.ikcd.net/HC5962-sysupgrade-20171221-b00a04d1.bin
2. Power off the router
3. Press and hold the reset button for 4~6 sec while power it back on
4. Connect a PC to router's LAN
5. Visit http://192.168.2.1 and upload the firmware

Then repeat the instruction in edae3479

 to install OpenWrt
Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>

target/linux/ramips/dts/mt7621_hiwifi_hc5962.dts

@@ -74,12 +74,12 @@
 
 		partition@140000 {
 			label = "kernel";
-			reg = <0x140000 0x200000>;
+			reg = <0x140000 0x400000>;
 		};
 
-		partition@340000 {
+		partition@540000 {
 			label = "ubi";
-			reg = <0x340000 0x1E00000>;
+			reg = <0x540000 0x1c00000>;
 		};
 
 		partition@2140000 {

target/linux/ramips/image/mt7621.mk

@@ -339,7 +339,7 @@ TARGET_DEVICES += gnubee_gb-pc2
 define Device/hiwifi_hc5962
   BLOCKSIZE := 128k
   PAGESIZE := 2048
-  KERNEL_SIZE := 2097152
+  KERNEL_SIZE := 4096k
   UBINIZE_OPTS := -E 5
   IMAGE_SIZE := 32768k
   IMAGES += factory.bin
@@ -349,9 +349,6 @@ define Device/hiwifi_hc5962
   DEVICE_VENDOR := HiWiFi
   DEVICE_MODEL := HC5962
   DEVICE_PACKAGES := kmod-mt7603 kmod-mt76x2 kmod-usb3 wpad-basic
-  SUPPORTED_DEVICES += hc5962
-  # Kernel partition too small
-  DEFAULT := n
 endef
 TARGET_DEVICES += hiwifi_hc5962