drivers/aspeed: Fix AST2400 POST failure without BMC FW or VBIOS

drivers/aspeed: Fix AST2400 POST failure without BMC FW or VBIOS

This patch is from Linux, commit 3856081eede2. The commit message there
is:

> commit 3856081eede297b617560b85e948cfb00bb395ec
> Author: Y.C. Chen <yc_chen@aspeedtech.com>
> Date:   Thu Feb 23 15:52:33 2017 +0800
>
>     drm/ast: Fix AST2400 POST failure without BMC FW or VBIOS
>
>     The current POST code for the AST2300/2400 family doesn't work properly
>     if the chip hasn't been initialized previously by either the BMC own FW
>     or the VBIOS. This fixes it.
>
>     Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
>     Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>     Tested-by: Y.C. Chen <yc_chen@aspeedtech.com>
>     Acked-by: Joel Stanley <joel@jms.id.au>
>     Cc: <stable@vger.kernel.org>
>     Signed-off-by: Dave Airlie <airlied@redhat.com>

Tested on a Supermicro X10SLM+-F with an AST 2400 where the BMC flash
chip has been completely erased. Before the patch, the display resembled
a rainbow. After the patch, the display works well.

Original-Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Original-Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Original-Tested-by: Y.C. Chen <yc_chen@aspeedtech.com>
Original-Signed-off-by: Dave Airlie <airlied@redhat.com>
Change-Id: I72efcf907fbd1263fe21d4f36fe900b305419c44
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/c/30507
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>