Add PulseAudio configuration for Librem 5 devkit

We need a custom script in order to set the default source and sink.

etc/pulse/daemon.conf.d/90-librem5-devkit.conf

+# We use this script to set the default source and sink
+default-script-file = /etc/pulse/librem5-devkit.pa

etc/pulse/librem5-devkit.pa

+.include /etc/pulse/default.pa
+
+set-default-sink alsa_output.platform-sound.VoiceCall__hw_CARD_sgtl5000__sink
+set-default-source alsa_input.platform-sound.VoiceCall__hw_CARD_sgtl5000__source