Use ids for identifying messages sent to EthosU firmware

Avoid to use pointers to kernel memory as identify messages, prior this
change, if memory is reused that can lead to unexpected conflicts.

Remove ref counting from capabilities and network_info since memory is
freed in only one place.

Finally, extract the capabilities code in its own files.

Change-Id: I9ab7257f3ab85fa3347be65b3739e0daf9b5a441
diff --git a/kernel/Kbuild b/kernel/Kbuild
index d54ec2c..d4610bc 100644
--- a/kernel/Kbuild
+++ b/kernel/Kbuild
@@ -28,4 +28,5 @@
                ethosu_network.o \
                ethosu_network_info.o \
                ethosu_watchdog.o \
+               ethosu_capabilities.o \
                ethosu_cancel_inference.o