linux-postmarketos-amlogic: upgrade to 6.8.4

To test:

• amediatech-x96-mini (@exkc)
• beelink-gt1 (@exkc) [Cannot be tested]
• librecomputer-lafrite (@funderscore)
• librecomputer-lepotato (@funderscore)
• librecomputer-solitude (@Newbyte, @funderscore)
• videostrong-kii-pro (@funderscore)
• xiaomi-once (@exkc) [Cannot be tested]