linux-postmarketos-rockchip: drop armv7 and upgrade to 6.0

Depends on !3524 (merged)

See !3524 (merged) for details about armv7 drop

Tested devices:

• google-kevin
• pine64-pinebookpro

CC @MartijnBraam (maintainer of linux-postmarketos-rockchip) @craftyguy (maintainer of pinebook pro)