linux-postmarketos-qcom-sdm845: upgrade to 6.9-rc4

we're long overdue an update, I fixed the fastrpc changes so @dylanvanassche 's patches work again, otherwise this is just a rebase, general stuff.

At this point things are bound to break, but that's why we have a great community who can help get stuff fixed. Unlike the attempted 6.7 upgrade I'd rather get this in and let folks fix it than ping the tested team and leave this in integration hell for the indefinite future...

boot tested on the op6.