It seems I could successfully compile and install the kernel by performing the following procedure
git clone --depth=1 https://github.com/raspberrypi/linux
cat patch-4.14.93-rt53.patch | patch -p1
sudo make modules_install
sudo cp arch/arm/boot/dts/*.dtb /boot/
sudo cp arch/arm/boot/dts/overlays/*.dtb* /boot/overlays/
sudo cp arch/arm/boot/dts/overlays/README /boot/overlays/
sudo cp arch/arm/boot/zImage /boot/$KERNEL.img
A reboot resulted in a non-bootable system. The following options added to cmdline.txt
made the system bootable again. I ran
sudo ./cyclictest -m -t1 -p 80 -n -i 500 -l 100000
from rt-tests and I get 15/20 ms latency.
The aim of this was to enhance the performance of sunvox, however I still have a very slow response from the program. But, the kernel-related issue is solved.