NEW BOARD: Dell OptiPlex 9020 MT (and 7020 MT)

Specifically the MT versions. The SFF versions will
be added separately, in a later commit.

See: https://review.coreboot.org/c/coreboot/+/55232


This patch has been added, from patchset 31. It still
has some unresolved issues, on that patchset, but
it should boot. See commit message there.

Of note: I've enabled PCI REBAR, though it's unknown
whether it will work (some comments there about it though,
on that gerrit page).

I've also set CBFS size to 8MB, not the full size of
the BIOS region; this is required on the T440p which
uses the same mrc.bin file, to get S3 working.

TSEG stage cache disabled, as on other Haswell boards.

The setup: SeaBIOS-only as first payload, but with GRUB
enabled as secondary payload. The _grubonly setup has
been enabled here. This way, the config will work on
iGPU and dGPU setups without issue.

Signed-off-by: Leah Rowe <info@minifree.org>