Add direct device reset function

The NPU API is only meant to be called from within the direct device, so
to make the reset functionality available for the rest of the driver a
direct device reset function has been added.

Change-Id: Ifef2f2fea92298d096ae71f8eecb5d34ac7e4963
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>