ramips: increase HiWiFi HC5962 kernel partition to 4M

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 edae3479e64e to install OpenWrt

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