[POWERPC] bestcomm: GenBD task support

This is the microcode for the GenBD task and the associated
support code. This is a generic task that copy data to/from
a hardware FIFO. This is currently locked to 32bits wide
access but could be extended as needed.

The microcode itself comes directly from the offical
API (v2.2)

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