linuxPackages.imx258: document why this custom kernel module is needed

and yes, this is still up-to-date with megi's as of linux 6.15

pkgs/linux-packages/imx258/package.nix

@@ -2,6 +2,8 @@
 # this is a patched version of mainline driver.
 # actually, mainline has diverged, so we lose some improvements at the benefit of others.
 # notably, megi fixed some purported power-on bugs.
+# the critical part of megi's patch appears to be changing the clock rate from 1272mbps -> 1224mbps.
+# "Pinephone Pro can't do 1272 mbps. This is just a hack to make the new upstream driver work quickly with the phone."
 {
   buildPackages,
   kernel,