soc/amd/picasso: only link soc_util in ramstage

No code that was or will be upstreamed uses functionality from soc_util
in romstage, so only compile and link it for ramstage.
This also allows to fix the SoC type detection in a follow-up patch
using information that FPS-M will be providing in a HOB.

BUG=b:153779573

Change-Id: If96e53608eadd562f6de5a0c370b89e84e43d049
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41430
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/amd/picasso/Makefile.inc

@@ -29,7 +29,6 @@ romstage-$(CONFIG_PICASSO_UART) += uart.c
 romstage-y += tsc_freq.c
 romstage-y += southbridge.c
 romstage-$(CONFIG_HAVE_SMI_HANDLER) += smi_util.c
-romstage-y += soc_util.c
 romstage-y += psp.c
 romstage-y += mtrr.c
 romstage-y += config.c