drm/ast: Base support for AST2500

Add detection and mode setting updates for AST2500 generation chip,
code originally from Aspeed and slightly reworked for coding style
mostly by Ben. This doesn't contain the BMC DRAM POST code which
is in a separate patch.

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