Separate generic and rpmsg device code

As a first step to support more NPU device types in the driver, the code
has been refactored to separate generic and rpmsg device code into their
own files.

Change-Id: Ie2f4feb47cef3081d1f764a313b602e0e7aa71c4
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>