[rpi3] switch to rpi4-mainline kernel

Switch to the linux-rpi4-mainline kernel, which is the 5.4 branch from Raspberry Pi, instead of using pure mainline.
Some stuff, like v3d, still does not work right on mainline, so using the rpi tree directly should enable that.

Signed-off-by: Dan Johansen <strit@manjaro.org>

devices/rpi3

 ## Maintained by Strit ##
 
 # Kernel and Bootloader stuff
-linux-aarch64
-uboot-raspberrypi
+linux-rpi4-mainline
 raspberrypi-bootloader
 bootsplash-theme-manjaro
 bootsplash-systemd