Change NPU kernel driver to use async probing

If the rpmsg channel to communicate with the subsystem is not available
until after the NPU kernel driver has been registered, the driver's
probe function will be called from the rpmsg message receive callback
and block handling of any other messages until the probing is done.

To avoid blocking the callback while performing the probing and to allow
the probe to communicate with the subsystem, the kernel driver will now
request that its probe function is called asynchronously.

Change-Id: Iaeaa4a2322f6a76c0acd202e69b91a8beeb6114b
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>