fsi: Add ast2600 master driver

The ast2600 BMC has a pair of FSI masters in it, behind an AHB to OPB
bridge.

The master driver supports reads and writes of full words, half word and
byte accesses to remote CFAMs. It can perform very basic error recovery
through resetting of the FSI port when an error is detected, and the
issuing of breaks and terms.

Signed-off-by: Joel Stanley <joel@jms.id.au>
--
v2:
 - remove debugging
 - squash in fixes