mb/intel/ptlrvp: Compile variant.c in ramstage for ptlrvp

In the mainboard directory for Intel's ptlrvp variant, ensure that the
`variant.c` file is compiled during the ramstage build process. This
adjustment enables the execution of variant_update_soc_chip_config()
for ptlrvp in ramstage, addressing CNVi Wi-Fi-related issues.

BUG=None
TEST=Verify that variant.c is compiled correctly in ramstage for
ptlrvp.

Change-Id: Iec9591aac536abfdd36dfba8a8fea689830ee41a
Signed-off-by: Bora Guvendik <bora.guvendik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87570
Reviewed-by: Zhixing Ma <zhixing.ma@intel.com>
Reviewed-by: Jamie Ryu <jamie.m.ryu@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Reviewed-by: Wonkyu Kim <wonkyu.kim@intel.com>
Reviewed-by: Cliff Huang <cliff.huang@intel.com>

src/mainboard/intel/ptlrvp/variants/ptlrvp/Makefile.mk

@@ -7,3 +7,4 @@ romstage-$(CONFIG_FW_CONFIG) += fw_config.c
 ramstage-y += gpio.c
 romstage-$(CONFIG_FW_CONFIG) += variant.c
 ramstage-$(CONFIG_FW_CONFIG) += fw_config.c
+ramstage-$(CONFIG_FW_CONFIG) += variant.c