drm/ast: POST code for the new AST2500

This is used when the BMC isn't running any code and thus has
to be initialized by the host.

The code originates from Aspeed (Y.C. Chen) and has been cleaned
up for coding style purposes by BenH.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>