Help on add a slightly different router -ramips mk7621 dts

hi people, i'm moving my first step on the source code side, im' not a coder, then sorry if i make some big mistake !!

i have a router MqMaker WITI board , this router is supported in the tree but only in the 256mb ram flavour , but my version is the 512 mb flavour

following this guide i change the code

i modify this file :

and add the file

where i change the memory address

you can see the change in my fork

i've tried it , and it works , but 2 question

when i make a sysupgrade , it give me an error

sysupgrade -i openwrt-ramips-mt7621-witi512-squashfs-sysupgrade.bin 
Device mqmaker,witi not supported by this image
Supported devices: witi512
Image check 'fwtool_check_image' failed

i think i've made some error with the name int the file i modified, but since i don't know very well what do every file i m unable to figure where is the problem

after correcting this problem, how i can submit to the standard tree ??