fsl-rio: Split rio driver into two parts, RapidIO endpoint and message unit

The Freescale PowerPC RapidIO controller consists of a RapidIO endpoint and
a RapidIO message unit(RMU). Or use RapidIO message manager(RMan) to
replace the RMU in DPAA architecture. Therefore, we should split the code
into two function modules according to the hardware architecture. Add new
struct for RMU module, and new initialization function to set up RMU
module. This policy is very conducive to adding new module like RMan, or
adding multi-ports or message units support.

Signed-off-by: Lian Minghuan <Minghuan.Lian@freescale.com>
Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Acked-by: Alexandre Bounine <alexandre.bounine@idt.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>