[POWERPC] bestcomm: core bestcomm support for Freescale MPC5200

This patch adds support for the core of the BestComm API
for the Freescale MPC5200(b). The BestComm engine is a
microcode-controlled / tasks-based DMA used by several
of the onchip devices.

Setting up the tasks / memory allocation and all common
low level functions are handled by this patch.
The specifics details of each tasks and their microcode
are split-out in separate patches.

This is not the official API, but a much cleaner one.
(hopefully)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>