nb/intel/sandybridge: Standardize MRC vs. native SPD mapping API

Changes both MRC and native raminit code path to get SPD mapping
from one place.

Boards with all memory socketed specify their mappings in a
devicetree setting introduced in commit 5709e03613b3
("nb/intel/sandybridge: Migrate MRC settings to devicetree") back in
May 2019 but remains unused as of this patch. This setting
will now hold raw SMBus addresses, and MRC raminit gets code to
translate them into a representation MRC expects.

Boards with soldered down memory (specifically with HAVE_SPD_IN_CBFS
in their board Kconfig), with or without socketed memory, specify
their layouts in mb_get_spd_map() as used by Haswell boards, where
they access hardware GPIO straps to select which SPD data to use.

This harmonizes the way boards specify their SPD layouts across
Haswell/SNB/IVB boards whether using MRC or native raminit. Going
forward they only need to specify the layout in one place. (Going
forward the devicetree setting should be backported to Haswell,
once we get native raminit working there.)

With this, northbridge code is now fully responsible for loading
all SPD data, be it from CBFS or SMBus.

To avoid breakage, transition will happen in stages:

1. This patch gets all the code in, and implements weak stubs that
maintain existing code and data flow (i.e. mainboards still populate
final SPD layout data). At this point devicetree already uses new
representation, but is still unused meaning no breakage.

2. Follow-up patch(es) remove mainboard_get_spd() from mainboards, and
replace it with mb_get_spd_map() or devicetree values (as appropriate)
with converted SPD info. The "weak" mainboard_get_spd() with new logic
takes over. Boards go Haswell Style at this point. Boards with MRC
raminit also lose code to fill in SPD data, allowing new data to take
hold.

3. Clean-up patch removes the weak functions and public prototypes re
mainboard_get_spd(), making it internal to northbridge. Changeover is
complete.

Change-Id: I1a75279d981e46505930a9ce1aae894ccc4e1f24
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76965
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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