mb/google/nissa/var/riven: Disable storage devices based on fw_config

Disable devices in variant.c instead of adding probe statements to
devicetree because storage devices need to be enabled when fw_config is
unprovisioned, and devicetree does not currently support this (it
disables all probed devices when fw_config is unprovisioned).

BUG=b:337169542
TEST=Local build successfully.

Change-Id: I3d71a35e9c0a33b72720b093b5a05eb69d5bb9f8
Signed-off-by: David Wu <david_wu@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83060
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
diff --git a/src/mainboard/google/brya/variants/riven/Makefile.mk b/src/mainboard/google/brya/variants/riven/Makefile.mk
index d38141c..86ba20d 100644
--- a/src/mainboard/google/brya/variants/riven/Makefile.mk
+++ b/src/mainboard/google/brya/variants/riven/Makefile.mk
@@ -3,4 +3,6 @@
 
 romstage-y += gpio.c
 
+ramstage-$(CONFIG_FW_CONFIG) += fw_config.c
+ramstage-$(CONFIG_FW_CONFIG) += variant.c
 ramstage-y += gpio.c