nb/intel/gm45: Split DDR2 JEDEC init out

Split JEDEC init into common and DDR3 specific parts and add the DDR2
specific init code. This also replaces raw `mchbar_clrsetbits32` calls
with a dedicated `jedec_command` function.

TEST: DDR2 systems boot (with the rest of the patch train)
- Tested on a Dell Latitude E6400
- Tested on a Compal JHL90
TEST: Ensure DDR3 systems still boot
- Tested on a Thinkpad X200

Change-Id: I7a57549887c0323e5babbf18f691183412a99ba9
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/34827
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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