broadwell: Flatten `mainboard_pre_raminit`

broadwell: Flatten `mainboard_pre_raminit`

All Broadwell boards only use the `mainboard_pre_raminit` function to
call `mainboard_fill_pei_data` and optionally `mainboard_fill_spd_data`.

Move the declaration and weak definition of `mainboard_fill_spd_data` to
platform code, replace the call to `mainboard_pre_raminit` in romstage.c
with calls to `mainboard_fill_pei_data` and `mainboard_fill_spd_data`,
and delete all other instances of `mainboard_pre_raminit` for Broadwell.
Finally, delete now-empty romstage.c and spd.h files from mainboards.

Change-Id: I3334b20bd7138bb753b996a137ff106e87c6e8a5
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49776
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>