* ramips: increase HiWiFi HC5962 kernel partition to 4M

Increase kernel partition because 2M is insufficient for 4.19
Because the partition changes, previous version of OpenWrt cannot upgrade to this version
Signed-off-by: NDENG Qingfang <dengqf6@mail2.sysu.edu.cn>

* ramips: use all reserved space for HiWiFi HC5962

Idea was from ad6c6361f9f8 ("ath79: enable all space on Netgear ar9344-based WNDR routers")

These stock partitons: "backup", "hw_panic", "overly", firmware_backup", "opt"
do not contain any device-specific data and can be used for /overlay, resulting in
121M space
Users should erase those partitions with command `mtd erase <partition name>` prior to
upgrading to this version
Signed-off-by: NDENG Qingfang <dengqf6@mail2.sysu.edu.cn>
Co-authored-by: NDENG Qingfang <dengqf6@mail2.sysu.edu.cn>