ramips: add new flash layout support to Phicomm K2

ramips: add new flash layout support to Phicomm K2

Descriptions：
Phicomm K2 (PSG1218) got a new "permanent_config" partition after
update firmware to v22.5. This partition located in front of the
firmware partition, same as The Phicomm K2P and K2G. Due to this
change the new bootloader can't load previous firmware any more.
This commit is aimed at add support for Phicomm K2 which official
firmware version is 22.5.x or newer. For which runs old firmware
version, just update OpenWrt that has a prefix of "k2-v22.4".
For uniform naming, this commit also changed the model name
PSG1218 to a more recognizable name K2, refer to Phicomm K2G,
K2P K2T.

OpenWrt selection table:
official firmware version           OpenWrt
v22.4.x.x or older              phicomm_k2-v22.4
v22.5.x.x or newer              phicomm_k2-v22.5

Installation:
Same as Phicomm K2G, K2P, PSG1208.
a. TFTP + U-Boot
b. Open telnet by some web page vulnerability (Search Baidu by key
   words "K2 telnet"), and then we can upload firmware image to
   /tmp and write it to firmware partition with mtd instruction.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
[rebase, add/harmonize version in model variables, fix version typo
 in commit message, wrap commit message properly]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>