skylake DDR4 boards: Set `CaVrefConfig` to 2

The `CaVrefConfig` FSP-M UPD describes how the on-die Vref generators
are connected to the DRAM. With the exception of an early Skylake RVP
board (which doesn't have coreboot support), mainboards using DDR3 or
LPDDR3 memory should set `CaVrefConfig` to 0, whereas mainboards with
DDR4 should set `CaVrefConfig` to 2. MRC uses this information during
memory training, so it is important to use the correct value to avoid
any issues, such as increased power usage, system instability or even
boot failures.

However, several Skylake DDR4 mainboards don't set `CaVrefConfig` to 2.
Although they can boot successfully, it's not optimal. For boards that
set `DIMM_SPD_SIZE` to 512 (DDR4 SPD size), set `CaVrefConfig` to 2.

Change-Id: Idab77daff311584b3e3061e9bf107c2fc1b7bdf1
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57262
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

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