remoteproc/pruss: split out PRU remoteproc driver from PRUSS subsystem driver

pruss_remoteproc.c includes two platform drivers, one manages PRU-ICSS
resources and the other is a remoteproc driver for the PRU cores in
the PRU-ICSS. Split these components out into logical files.

This uses the minimal changes required without breaking build or
functionality. A lot of structures have been moved into the
pruss.h header that will be moved back out when new API do get
introduced in the following patches.

Signed-off-by: Andrew F. Davis <afd@ti.com>
[s-anna@ti.com: header split between pru_rproc.h and pruss.h]
Signed-off-by: Suman Anna <s-anna@ti.com>