util/mb/google: Update templates to refer to the new spd_tools

util/mb/google: Update templates to refer to the new spd_tools

Update the new variant templates to refer to the new unified version
of spd_tools:
- Update the comments in mem_parts_used.txt
- Change the placeholder SPD in Makefile.inc to 'placeholder'

BUG=b:191776301
TEST=None

Change-Id: I03265de0d1182da81dd25a2fe6f940a0b82e5fa4
Signed-off-by: Reka Norman <rekanorman@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57774
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>