[mrf24j40] Use -arch driver for platform code

Compile tested only.  I took the simplest path to separating the arch code
from the independent radio driver code, but this probably will result in
bigger/slower code.  One day, compilers will make it all irrelevant.

However, this paves the way for getting a second user of the common code
[mrf24j40] Flag a probable error in existing code

I'm not trying to bugfix right now, just refactor.