Create device for Ethos-U kernel driver

When the Ethos-U kernel driver is probed it creates a /dev/ethosu<nr>
device node, which user space use ioctl to communicate with. When
the driver is removed the Ethos-U resources must live on until all
open file handles have been closed.

The rpmsg device can be removed for a number of reasons, for example
if the firmware is stopped or the remoteproc driver is removed. To
allow the remove to complete without waiting for all file handles to
close, a new 'struct device' is created by the kernel driver. This
device will be used to memory allocations and will live on until the
last file handle has been closed.

Change-Id: I790d8219960f25fe64f58c11a865eb65c7b08974